单选题Dr. Beatrice Hahn of the University of Alabama at Birmingham announced that she" d learned the origins of HIV-1, the virus responsible for 99 percent of the world" s 33million AIDS cases. Hahn and her colleagues were able to trace it specifically to a subspecies of chimpanzee called Pan troglodytes. Chimps are not the only primates that carry viruses related to HIV. Earlier this decade scientists linked HIV-2 to a monkey called the sooty mangabey. Mandrills and African green monkeys carry still other simian immuno-deficiency viruses(SIVs). The closest links to HIV-1 seemed to be three viral samples isolated in chimpanzees. But one of the three was so divergent from the other two that scientists doubted they were HIV-1" s immediate ancestors. To solve this puzzle, Hahn and her international team first had to fit the human and simian viruses into genetic "family trees" that accounted for their similarities and differences. They next had to show that the chimps were in the same region where AIDS got its start in humans. In fact, the chimp subspecies lives in West Central Africa, the region where HIV-1 is thought to have originated. There was even a plausible mode of transmission from chimp to human. The animals have long been hunted for food, so blood from the carcasses could easily have entered the hunters" bodies through superficial wounds. Hahn"s group showed how, after jumping species on at least three occasions, chimpanzee SIV evolved into the three families of HIV-1 strains recognized today. The question is not merely of academic interest. The chimps, it seems, carry their version of the virus but do not get sick from it. In theory at least, they could reveal important clues about controlling AIDS. "Humans share 98.5 percent of their genes with chimpanzees, points out Dr Feng Gao of UAB, one of Hahn" s collaborators. But just as scientists are learning the chimps" value in AIDS research, this critical subpopulation is being hunted to the brink of extinction. Not only does that mean hunters may be contracting additional viral strains; it also means that mankind is losing a living data-base of AIDS information. "It" s like burning a whole library of books without having read them," says Hahn. Given the similarity of man and chimp, it could prove a tragic loss. Who knows what other mysteries may be answered in those books.

单选题Cholesterol plays a key role in the development of embryo cells, biologists have discovered. They say that this much maligned fatty molecule, best known for its links to heart disease, helps to position a family of "controller" proteins that tell embryo cells how they should form. In laboratory studies on fruit flies and mice, Phil Beachy and his colleagues at Johns Hopkins University in Baltimore looked at members of a family of controller proteins known as Hedgehog. Fruit fly embryos that lack the genes for these proteins develop a spiky appearance — hence the name. Biologists already know that Hedgehog proteins help to direct the development of embryo cells by delivering signals at the cell surface. The proteins only deliver their signals when they split in two, and one half binds to a lipid, a fatty molecule. Beachy" s team discovered that this lipid is cholesterol. In previous test-tube experiments, scientists have found that the concentration and distribution of Hedgehog proteins on the surface of embryo cells can be critical in determining their fate. For example, cells taken from the embryonic nervous system become motor neurons when exposed to low concentrations of Hedgehog proteins, while at high concentrations they turn into structural cells known as floor plates. Until now, however, no one knew what determined the concentration of the proteins on the surface of cells in the embryo. Beachy" s findings suggest that cholesterol" s role is to allow the signaling proteins to enter cells. Since the lipid molecule is water-hating or hydrophobic, the researchers argue, it may act as a key, allowing the Hedgehog proteins to move through the hydrophobic membranes of target cells, or to stick to them. The results appear in the current issue of Science. Researchers already know that cholesterol does some important jobs: for example, it is involved in the production of hormones and is vital for fertility and for the healthy membrane structure of cells. Beachy" s latest results show that cholesterol is also needed for embryonic development. They also help to explain a long-known but mysterious finding that pregnant rats given drugs that block the manufacture of cholesterol produce deformed fetuses. Gail Martin, a developmental biologist at the University of California, San Francisco, describes the findings as "exciting". They may also have wider implications, she says: it is likely that cholesterol" s action is needed more generally for signaling between cells. Beachy too believes that the findings may have "strong connections for human biology". For example, they throw light on a birth disorder that has been linked to the mother" s inability to make cholesterol. This disorder, Smith-Lemli-Opitz syndrome, affects about 1 in 9000 newborn babies, causing defects such as small head size.

单选题Every year, the American Lung Association(ALA)releases its annual report card on smog, and every year it gives "F" to over half the nation"s counties and cities. When ALA"s "state of the Air 2002" recently came out, dozens of credulous local journalists once again took the habit, ominously reporting that their corner of the nation received a failing grade. The national coverage was no better, repeating as fact ALA"s statement that it is "gravely concerned" about air quality, and neglecting to solicit the views of even one scientist with a differing view. Too bad, because this report card says a lot less about actual air quality than it does about the tactics and motives of the ALA. The very fact that 60 percent of counties were given an "F" seems to be alarmist. This is particularly true given that smog levels have been trending downward for several decades. According to the Environmental Protection Agency(EPA)statistic, ozone, the primary constituent of smog, has declined by approximately 30 percent since the 1970s. And recent gains indicate that the progress will likely continue, even without the wave of new regulations ALA is now demanding. ALA is correct that some areas still occasionally exceed the federal standard for ozone, but such spikes are far less frequent than in the past. Even Los Angeles, the undisputed smog capital of America, has cleaned up its act considerably. Los Angles, which exceeded federal smog standards for 154 days in 1989, has had 75 percent fewer such spikes in recent years. But an ALA-assigned "F" misleadingly implies that air quality are not improved at all. Most of the nation currently in attainment with the current smog standard, and much of the rest is getting close. Nonetheless, ALA chose to assign an "F" to an entire country based on just a few readings above a strict new EPA standard enacted in 1997 but not yet in force. In effect, ALA demanded a standard even more stringent than the federal government"s, which allows some leeway for a few anomalously high readings in otherwise clean areas. ALA further exaggerated the public health hazard by grossly overstating the risk of these relatively minor and sporadic increases above standard.

单选题Physics and chemistry have furnished the techniques for biologists to take the exploration of life to its logical conclusion. This event demands a wider audience. The pattern of discovery cannot be predicted, but the tool kit now in the possession of biologists is so powerful that no known problem seems likely to hold its mysteries for long. By usurping the tools with which living cells manipulate DNA, biologists can cut the genetic material at chosen points and amplify target segments for further study. Techniques and software for sequencing the order of chemical units in DNA are now so sophisticated that the full human gene set seems likely to be sequenced within the next ten years. It is only a matter of time before biologists catalogue the 75 ,000 or so human genes; identify the signals that switch each gene on and off; figure out what sets of active genes characterize each of the 200 or so major types of human cells, and determine how the consortium of interacting cells, and determine how the consortium of interacting cell types operates as an organism. Such knowledge has larger consequences than scientists can deal with alone. Genes do not determine everything, but they set the boundaries of an individual' s full potential. They probably decree major elements of personality and intelligence. They create a predisposition to various diseases. Knowledge of how the human machine is assembled implies the knowledge to repair, refine and improve it. The first attempts at gene therapy have mostly stumbled, but the technique will surely be made to work eventually. The idea of correcting defective genes is not particularly controversial. But a fundamental solution to the creeping burden of Medicare—equipping every embryo with a birthright package of genes for good health, longevity and aversion to nicotine and violence—would raise knottier issues. Critics will doubtlessly warn darkly of dangerous knowledge. But more knowledge is generally better than less. Molecular biology, long gathering speed, is about to take off. It is almost ready to become a spectator sport, not just a private club for academics and biotechnology companies.

单选题In the college-admissions wars, we parents are the true fighters. We are pushing our kids to get good grades, take SAT preparatory courses and build resumes so they can get into the college of our first choice. I"ve twice been to the wars, and as I survey the battlefield, something different is happening. We see our kids" college background as a prize demonstrating how well we"ve raised them. But we can"t acknowledge that our obsession(痴迷)is more about us than them. So we"ve contrived various justifications that turn out to be half-truths, prejudices or myths. It actually doesn"t matter much whether Aaron and Nicole go to Stanford. We have a full-blown prestige panic; we worry that there won"t be enough prizes to go around. Fearful parents urge their children to apply to more schools than ever. Underlying the hysteria(歇斯底里)is the belief that scarce elite degrees must be highly valuable. Their graduates must enjoy more success because they get a better education and develop better contacts. All that is plausible— and mostly wrong. We haven"t found any convincing evidence that selectivity or prestige matters. Selective schools don"t systematically employ better instructional approaches than less selective schools. On two measures — professors" feedback and the number of essay exams — selective schools do slightly worse. By some studies, selective schools do enhance their graduates" lifetime earnings. The gain is reckoned at 2-4% for every 100-point increase in a school"s average SAT scores. But even this advantage is probably a statistical fluke(偶然). A well-known study examined students who got into highly selective schools and then went elsewhere. They earned just as much as graduates from higher-status schools. Kids count more than their college. Getting into Yale may signify intelligence, talent and ambition. But it"s not the only indicator and, paradoxically, its significance is declining. The reason: so many similar people go elsewhere. Getting into college is not life only competition. Old-boy networks are breaking down. Princeton economist Alan Krueger studied admissions to one top Ph.D. program. High scores on the GRE helped explain who got in; degrees of prestigious universities didn"t. So, parents lighten up. The stakes have been vastly exaggerated up to a point, we can rationalize our pushiness. America is a competitive society; our kids need to adjust to that. But too much pushiness can be destructive. The very ambition we impose on our children may get some into Harvard but may also set them up for disappointment. One study found that, other things being equal, graduates of highly selective schools experienced more job dissatisfaction. They may have been so conditioned to being on top that anything less disappoints.

单选题You are busy filling out the application form for a position you really need; let" s assume you once actually completed a couple of years of college work or even that you completed your degree. Isn" t it tempting to lie just a little, to claim on the form that your diploma represents a Harvard degree? Or that you finished an extra couple of years back at State University? More and more people are resorting to outright deception like this to land their first job or to move ahead in their careers, for personnel officers, like most Americans, value degrees from prestigious schools. A job applicant may have a good education anyway, but he or she assumes that chances of being hired are better with a diploma from a well-known university. Registrars at most well-known colleges say they deal with fraudulent claims like these at the rate of about one per week. Personnel officers do check up on degrees listed on application forms, then. If it turns out that an applicant is lying, most colleges are reluctant to accuse the applicant directly. One Ivy League school calls them " impostors" ; another refers to them as " special cases". One well-known West Coast school, in perhaps the most delicate of all, says that these claims are made by " no such people" . To avoid outright lies, some job-seekers claim that they "attended" or "were associated with" a college or university. After carefully checking, a personnel officer may discover that "attending" means flunking out after one semester. It may tie that "being associated with" a college means that the job-seeker visited his younger brother for a football weekend. One school that keeps records of false claims says that the practice dates back at least to the turn of the century—that" s when they began keeping records, anyhow. If you don" t want to lie or even stretch the truth, there are companies that will sell you a phon-y diploma. One company, with offices in New York and on the West Coast, will put your name on a diploma from any number of nonexistent colleges. The price begins at around twenty dollars for a diploma from "Smoot State University". The prices increase rapidly for a degree from the "University of Purdue". As there is no Smoot State and the real school in Indiana is properly called Purdue University, the prices seem rather high for one sheet of paper.

单选题Some futurologists have assumed that the vast upsurge of women in the workforce may portend a rejection of marriage. Many women, according to this hypothesis, would rather work than marry. The converse of this concern is that the prospects of becoming a multi-paycheck household could encourage marriage. In the past, only the earnings and financial prospects of the man counted in the marriage decision. Now, however, the earning ability of a woman can make her more attractive as a marriage partner. Data show that economic downturns tend to postpone marriage because the parties cannot afford to establish a family or are concerned about rainy days ahead. As the economy rebounds, the number of marriages also rises. Coincident with the increase in women working outside the home is the increase in divorce rates. Yet, it may be wrong to jump to any simple cause-and-effect conclusions. The impact of a wife"s work on divorce is no less cloudy than its impact on marriage decisions. The realization that she can be a good provider may increase the chances that a working wife will choose divorce over an unsatisfactory marriage. But the reverse is equally plausible. Tensions grounded in financial problems often play a key role in ending a marriage. Given high unemployment, inflationary problems, and slow growth in real earnings, a working wife can increase household income and relieve some of these pressing financial burdens. By raising a family"s standard of living, a working wife may strengthen her family"s financial and emotional stability. Psychological factors also should be considered. For example, a wife blocked from a career outside the home may feel caged in the house. She may view her only choice as seeking a divorce. On the other hand, if she can find fulfillment through work outside the home, work and marriage can go together to create a stronger and more stable union. Also, a major part of women"s inequality in marriage has been due to the fact that, in most cases, men have remained the main breadwinners. With higher earning capacity and status occupations outside of the home comes the capacity to exercise power within the family. A working wife may rob a husband of being the master of the house. Depending upon how the couple reacts to these new conditions, it could create a stronger equal partnership or it could create new insecurities.

单选题Visual impairment carries with【C1】______ ability to travel through one's physical and social until adequate orientation and mobility skills have been established. Because observational skills are more limited, self-control within the immediate surroundings is limited. The visually impaired person is less able to anticipate【C2】______ situations or obstacles to avoid. Orientation refers to the【C3】______ map one has of one's surroundings and to the relationship between self and that environment. It is best generated by moving through the environment and【C4】______ together relationships, object by object, in an organized approach. With【C5】______ visual feedback to reinforce this map, a visually impaired person must rely on memory for key landmarks and other clues, which enable visually impaired persons to【C6】______ their position in space. Mobility is the ability to travel safely and efficiently from one point to another within one's physical and social environment. Good orientation skills are necessary to good mobility skills. Once visually impaired students learn to travel safely as pedestrians (行人), they also need to learn to use public transportation to become as【C7】______ as possible. To meet the【C8】______ demands of the visually impaired person, there is a sequence instruction that begins during the preschool years and may continue after high schools. Many visually impaired children lack adequate concepts regarding time and space or objects and events in their environment. During the early years much attention is focused on the development of some fundamental【C9】______ , such as inside or outside, in front of or behind, fast or slow, which are essential to safe,【C10】______ travel through familiar and unfamiliar settings.

单选题Women find a masculine face—with a large jaw and a prominent brow—-more attractive when they are most likely to attractive, according to a study published in the June 24 NATURE. Before, during, and use after menstruation, however, they seem to be drawn to less angular, more "feminine" male faces, the researchers report. " Other studies of female preference, mainly for odors, show changes across the menstrual cycle ," says lead author Ian Penton-Voak of the University of St. Andrews in Scotland. " We thought it would be interesting to look at visual preferences and see if they changed also". The researchers showed 39 Japanese women composite male faces that emphasized masculine or feminine facial features to differing degrees. The women preferred images with more masculine features when they were in the fertile phase of their menses but favored more feminine features during their less fertile phase. The type of face women find attractive also seems to depend on the kind of relationship they wish to pursue, according to another experiment. The cyclic preference for muscular faces was evident among 23 British women asked to choose the most attractive face for a short-term relationship, Penton-Voak says. The 26 women asked to choose an attractive face for a long-term relationship, however, preferred the more feminine features throughout their menstrual cycle. Another 22 women who were using oral contraceptives did not show monthly changes in the faces they preferred even for short-term relationships, indicating that hormones might play a role in determining attractiveness, Penton-Voak says. Men whose faces have some feminine softness are perceived as " kinder" men who may make better husbands and partners, he adds, while macho features may be associated with higher testosterone(睾丸素)levels and good genes. He cautions, however, that research hasn"t yet shown a link between a woman" s preferences in such tests and her actual behavior.

单选题Although biological mechanisms do not work with the accuracy or stability of modern clocks, a sense of time and its rhythm is built into the functioning of the human body. Our heart, with its beating pulse, is the clocklike internal rhythm of which we are most aware. In his discovery of the law of the pendulum, which turned out to have the most profound effect on all later time—measuring devices, Galileo used—if legend can be believed—his own pulse beat as the test. There are, however, other biological timekeepers that play important roles in our lives. These inner clocks are generally very regular, but they can also be "reset" and will fall in step with a shifted rhythm. Even after we take a long flight across the Atlantic or Pacific, our lack of synchronization with the local time slowly disappears. The technical term, introduced in 1959, for the internal timer that keeps track of this 24-hour periodicity and retains it even in the absence of external cues is the circadian system (from the Latin circa for "about" or "approximately" and dies for "day"). Though known to biologists for over 200 years, biological clocks have been the subject of intensive research during the last half century. The first human physiological variables that scientists observed to be governed by a circadian rhythm were pulse rate and body temperature. Even if a person rests in bed and fasts, his or her deep-body temperature will vary by almost one degree centigrade between its low in the early morning hours and a high late in the afternoon. More than 100 additional physiological and psychological variables are also subject to diurnal periodicities. For example, the speed with which children can do computations varies by about 10 percent between its slowest value in the early morning to a high before noon, dropping to a nadir in the early afternoon, rising again to a peak at about 6 o' clock and then falling off in the evening. This pattern was first measured in 1907 and replicated a half century later. The extremely controversial question that arose immediately was to what extent this human circadian rhythm was an autonomous mechanism rather than a simple response to external signals, such as changes in the level of light, the times of meals, or social interactions with our surroundings. It has not been easy to find the answer, but careful laboratory experiments have led to the definite conclusion that our body contains an autonomous timekeeper. Individuals who volunteered to be kept in artificial isolation with no time cues of any kind also helped find the answer. In 1962 a French researcher spent two months in a cold cave, 375 feet underground in the Alps. The Frenchman called his aboveground supporters by telephone whenever he ate, went to sleep, and woke, and he recorded in detail his thoughts and impressions of the passage of time. He and all such explorers found themselves subject to definite internal time signals. It turned out, however, that the measured period of their bodily variables (all of which were consistent with one another), as well as their subjective impression of the time of day and their periods of sleep and waking , was slightly longer than 25 hours. By the time they emerged from their prolonged isolation, their internal timer was many hours out of phase with the external 24-hour clock. Today, the autonomy of biological clocks is a well-established fact. Though running at a steady rate, our internal clock is "slow" by about an hour per day, but since it is continually automatically reset by cycles of light and dark, under normal circumstances the loss of time is not cumulative, our internal clock is entrained with the rhyme of the Sun.

单选题Cancer is the second leading cause of death in the United States, after heart disease. In the past, it was often considered a death sentence. But many patients now live longer【C1】______of improvements in discovery and treatment. Researchers say death【C2】______in the United States from all cancers combined have fallen for thirty years. Survival rates have increased for most of the top fifteen cancers in both men and women, and for cancers in【C3】______. The National Cancer Institute and the Centers for Disease Control and Prevention studied the number of cancer survivors. A cancer survivor is defined【C4】______anyone who has been found to have cancer. This would include current patients. The study covered the period from 1971 to 2001. The researchers found there are three【C5】______as many cancer survivors today as there were thirty years ago. In 1971, the United States had about threemillion cancer survivors. Today there are about tenmillion. The study also found that 64% of adults with cancer can expect to still be【C6】______in five years. Thirty years ago, the five-year survival rate was 50%. The government wants to【C7】______the five-year survival rate to 70% by 2010. The risk of cancer increases with age. The report says the majority of survivors are 65 years and older. But it says medical improvements have also helped children with cancer live【C8】______longer. Researchers say 80% of children with cancer will survive at least five years after the discovery. About 75% will survive at【C9】______ten years. In the 1970s, the five-year survival rate for children was about 50%. In the 1960s, most children did not survive cancer. Researchers say they expect more improvements in cancer treatment in the future. In fact, they say traditional cancer-prevention programs are not enough anymore. They say public health programs should also aim to support the【C10】______numbers of cancer survivors and their families.

单选题I was invited to present a lecture to a class of graduate nurses who were studying the " Psychosocial Aspects of Aging. " I started my lecture with the following case presentation: The patient is a white female who appears her reported age. She neither speaks nor comprehends the spoken word. Sometimes she babbles incoherently for hours on end. She is disoriented about person, place, and time. She does, however, seem to recognize her own name. I have worked with her for the past 6 months, but she still does not recognize me. She shows complete disregard for her physical appearance and makes no effort whatsoever to assist in her own care, she must be fed, bathed, and clothed by others. Because she is edentulous, her food must be pureed, and because she is incontinent of both urine and stool, she must be changed and bathed often. Her shirt is generally soiled from almost incessant drooling. She does not walk. Her sleep pattern is erratic. Often she awakens in the middle of the night, and her screaming awakens others. Most of the time she is friendly and happy. However, several times a day she gets quite agitated without apparent cause. Then she screams loudly until someone comes to comfort her. After the case presentation, I asked the nurses how they would feel about taking care of a patient such as the one described. They used words such as " frustrated," " hopeless, " " depressed, " and "annoyed" to describe how they would feel. When I stated that I enjoyed taking care of her and that I thought they would too, the class looked at me in disbelief. I then passed around a picture of the patient; my 6-month-old daughter. After the laughter had subsided, I asked why it was so much more difficult to care for a 90-year-old than a 6-month-old with identical symptoms. We all agreed that it is physically easier to take care of a helpless baby weighing 15 pounds than a helpless adult weighing 100, but the answer seemed to go deeper than that. The infant, we all agreed, represents new life, hope, and almost infinite potential. The demented senior citizen, on the other hand, represents the end of life, with little potential for growth. We need to change our perspective. The aged patient is just as lovable as the child. Those who are ending their lives in the helplessness of old age deserve the same care and attention as those who are beginning their lives in the helplessness of infancy.

单选题A mother who is suffering from cancer can pass on the disease to her unborn child in extremely rare cases,【C1】______a new case report published in PNAS this week. According to researchers in Japan and at the Institute for Cancer Research in Sutton, UK, a Japanese mother had been diagnosed with leukemia a few weeks after giving birth,【C2】______tumors were discovered in her daughter's cheek and lung when she was 11 months old. Genetic analysis showed that the baby's cancer cells had the same mutation as the cancer cells of the mother. But the cancer cells contained no DNA whatsoever from the father,【C3】______would be expected if she had inherited the cancer from conception. That suggests the cancer cells made it into the unborn child's body across the placental barrier. The Guardian claimed this to be the first【C4】______case of cells crossing the placental barrier. But this is not the case — microchimerism,【C5】______cells are exchanged between a mother and her unborn child, is thought to be quite common, with some cells thought to pass from fetus to mother in about 50 to 75 per cent of cases and to go the other way about half【C6】______. As the BBC pointed out, the greater【C7】______in cancer transmission from mother to fetus had been how cancer cells that have slipped through the placental barrier could survive in the fetus without being killed by its immune system. The answer, in this case at least, lies in a second mutation of the cancer cells, which led to the【C8】______of the specific features that would have allowed the fetal immune system to detect the cells as foreign. As a result, no attack against the invaders was launched. 【C9】______, according to the researchers there is little reason for concern of "cancer danger". Only 17 probable cases have been reported worldwide and the combined【C10】______of cancer cells both passing the placental barrier and having the right mutation to evade the baby's immune system is extremely low.

单选题Some futurologists have assumed that the vast upsurge of women in the workforce may portend a rejection of marriage. Many women, according to this hypothesis, would rather work than marry. The converse of this concern is that the prospects of becoming a multi-paycheck household could encourage marriage. In the past, only the earnings and financial prospects of the man counted in the marriage decision. Now, however, the earning ability of a woman can make her more attractive as a marriage partner. Data show that economic downturns tend to postpone marriage because the parties cannot afford to establish a family or are concerned about rainy days ahead. As the economy rebounds, the number of marriages also rises. Coincident with the increase in women working outside the home is the increase in divorce rates. Yet, it may be wrong to jump to any simple cause-and-effect conclusions. The impact of a wife's work on divorce is no less cloudy than its impact on marriage decisions. The realization that she can be a good provider may increase the chances that a working wife will choose divorce over an unsatisfactory marriage. But the reverse is equally plausible. Tensions grounded in financial problems often play a key role in ending a marriage. Given high unemployment, inflationary problems, and slow growth in real earnings, a working wife can increase household income and relieve some of these pressing financial burdens. By raising a family's standard of living, a working wife may strengthen her family's financial and emotional stability. Psychological factors also should be considered. For example, a wife blocked from a career outside the home may feel caged in the house. She may view her only choice as seeking a divorce. On the other hand, if she can find fulfillment through work outside the home, work and marriage can go together to create a stronger and more stable union. Also, a major part of women's inequality in marriage has been due to the fact that, in most cases, men have remained the main breadwinners. With higher earning capacity and status occupations outside of the home comes the capacity to exercise power within the family. A working wife may rob a husband of being the master of the house. Depending upon how the couple reacts to these new conditions, it could create a stronger equal partnership or it could create new insecurities.

单选题WELL—do they or don"t they? For years, controversy has raged over whether the electromagnetic fields produced by power lines could cause cancer especially leukemia in young children. But in Britain last week confusion reached new heights. One team from Bristol announced that it had evidence to back a controversial but plausible theory which would explain how power lines might cause cancer(electric fields attract airborne pollutants). Only to be followed by the release of results by another group in London which suggested there is nothing to worry about. What is going on? Actually, the confusion may be more apparent than real. There can be no doubt that the effects of power lines on water droplets, pollutants and naturally occurring radon uncovered by the Bristol team are real and interning. But to suggest that they have anything to do with leukemia in children is premature. The extra exposure to pollution for a child living near power lines would be tiny, and it is not obvious why radon, a gas normally associated with lung cancer—would cause leukemia in children. The second study, which drew reassuring blank, is the world" s biggest ever probe of the statistical link between childhood cancers and magnetic fields of the sort produced by power lines and electrical appliances. It is one of several recent studies that have failed to find a link. Unlike earlier research, these newer studies involved going into homes to measure the electromagnetic fields. The fields they measured included input from major power lines if they were. Which is not to say the research is perfectly. Critics argue that Britain" s childhood cancer study, for example, has not yet taken into account the surges in exposure that might come from, say, switching appliances on and off. And some people might wonder why measurements of the electric fields that are also produced by power lines did not figure in last week" s study. But neither criticism amounts to a fatal blow. Electrical fields cannot penetrate the body significantly, for example. A more serious concern is whether the British research provides an all-clear signal for such countries as the US where power lines carry more current and therefore produce higher magnetic fields. Pedants(书呆子)would conclude that it doesn" t. But these counties will not have long to wait for answers from a major Japanese study. In Britain the latest epidemiological study can be taken as the final word on the matter. If the electromagnetic fields in British homes can in some unforeseen way increase the risk of cancer, we can now be as certain as science allows that the increase is too tiny to measure.

单选题The United States and England each has a major—and unique—health-care challenge, according to a study comparing the health of senior citizens in the two countries. The study, conducted by researchers from RAND Corporation in the United States and Institute for Fiscal Studies in the United Kingdom, found that disease and health disorder incidence was higher among U. S. senior citizens, but mortality rates were higher among English senior citizens. Americans aged 65 and older have almost twice the rate of diabetes found among their English counterparts and more than double the rate of cancer. Nevertheless, death rate among Americans 65 and older is lower. "Americans are a sicker group of people who tend to live longer, " says James Smith, a study co-author. He attributes the U. S. health problems to lifestyle factors, including poor eating habits and inadequate exercise. Americans tend to eat much larger servings of food, for example, "There is what I call an American plate. When we go to a restaurant, it's a plate I can't even eat any more. It's a plate with so much food on if it's not even appealing to me. " Smith also says that English adults are generally much more physically active than Americans. Biking and walking are much common in everyday life in England. He observes that "there is a lot of walking in London, and there is a lot of bicycle riding. I don't see people in downtown Los Angeles on their bicycles". On the other hand, England's problem is that doctors fail to diagnose serious conditions early enough. American doctors tend to screen patients for cancer, diabetes, and other illnesses more frequently. Smith notes "American medicine is much more aggressive. It leads to high costs, but it has benefits, too".

单选题Fourteen-year-old Sean MeCallum lay in a hospital bed waiting for a new heart. Without it, Scan would die. Sean" s case is not unusual. Everyday many people die because there just aren" t enough human organs to go around. Now scientists say they can alter the genetic make-up of certain animals so that their organs may be acceptable to humans. With this gene-altering technique to overcome our immune rejection to foreign organs, scientists hope to use pig hearts for transplants by the year 2008. That prospect, however, has stirred up strong opposition among animal fight activists. They protest that the whole idea of using animal organs is cruel and unjust; some scientists also fear such transplants may transform unknown diseases to humans. Others believe transplanting animal organs into humans is unnecessary. Millions of dollars spent on breeding pigs for their organs could be better spent on health education programs. They believe seventy-five percent of the heart disease cases that lead to a need for organ transplant are preventable. The key is to convince people to eat healthfully, and not to smoke or drink alcohol. Scientists could also use research funds to improve artificial organs. Still others believe that though new inventions and prevention programs may help, spending money to encourage more people to donate their organs is an even better idea. If enough people were educated about organ donations, everyone who needed an organ could be taken off the waiting list in a year.

单选题English speakers pick up pitch in the right hemispheres of their brains, but speakers of certain other languages perceive it on the left as well. It all depends on what you want to learn from pitch, Donald Wong of the Indiana School of Medicine in Indianapolis told the meeting last week. Earlier studies have shown that when an English speaker hears pitch changes, the right prefrontal cortex leaps into action. This fits in with the idea that emotive nuances of language—which in English are often carried by the rise and fail of the voice—are perceived on the right. But in "tonal" languages like Thai, Mandarin and Swedish, pitch not only carries emotional information, but can also alter the meaning of a word. Wong and his colleagues suspected that a speaker of tonal language would register pitch in the left side of the brain—in particular Broca" s area, which processes the linguistic content of language. To test this, the team asked English speakers and Thai speakers to listen to 80 pairs of Thai words, and tracked the blood flow in their brains using positron emission tomography. The volunteers had to decide whether the two words sounded the same, either by consonant or by tone. In some eases, the words had no intelligible meaning. None of the words was emotionally charged, so even when Thai speakers could understand them, there was no right-side activation. But sure enough the Thai speakers consistently lit up the left side of the brain, especially Broca" s area, while the English speakers did not. The researchers are now planning to repeat the experiment with Thai speakers using whole sentences , complete with emotional information. " Both hemispheres will be engaged, " predicts Wong.

单选题Monkeys and chimpanzees, although they are weaker and less fierce than many other animals, possess brains which are far along the evolutionary road as any creature other than man. Birds can perform marvels of aerobatics, they can catch insects on the wing with unparalleled skill, navigate in a remarkable manner half round the world and back—but they can cannot think and reason. In technical terms it can be said that they are lacking in insight. The abilities which they do possess are built-in instincts derived from their genetic inheritance. Monkeys, on the other hand, can reason. They can easily remember a lighted door indicating the presence of food. They can remember what kind of food they are looking for. A monkey set the problem of reaching a banana, say, hung high up in its cage, can work out a system for getting it even if it involves piling up boxes to stand on and then knocking down the banana with a stick. A charming story is told about the psychologist Wolfgang Kobler, who had provided various boxes and other apparatuses by which he proposed to test a chimpanzee's ability to think out a method of reaching a fruit hung nine feet in the air. The animal looked about it and sized up the problem. Then it took Kobler by the hand, led him to a position immediately under the banana, jumped upon to his shoulder and reached it down from there. But evolution, although it has brought monkeys to a remarkable degree of cleverness, has stopped short at a crucial ability, the possession of which places man at a clearly superior level. Their minds cannot cope with abstract ideas. For example, an ape can be taught to fill a can with water from a barrel and take the can of water to put out a fire so that it can reach into a box and get food. But if the whole set-up is arranged on a raft the animal will continue to draw its water only from the barrel. It cannot grasp that any water, taken more, conveniently, say, from the pond on which the raft is floating, will putout the fire just as well. The abstract idea that water puts out fire is beyond it. The abstract idea that water puts out fire is beyond it.

单选题Memory can be both enhanced and impaired by the use of drugs. To date, most of the research in this area has focused on the short- and long-term effects of alcohol. Overall, the results show that alcohol interferes with the ability to encode new information but has relatively little effect on the retrieval of data already stored in long-term memory(Bimbaum et al., 1978). Heavy use of alcohol, however, may result in significant memory loss. In some cases, everything that was said or done while intoxicated is completely blocked out of memory. The person may fail to remember even highly significant events - such as having injured somebody! In extreme cases, chronic alcoholism may result in Korsakoff" s psychosis, a disorder characterized by gross memory defects and disorientation. Sufferers of Korsakoff" s psychosis may lose track of their own names, where they came from, or where they are. The effects of marijuana on memory are far less clear. In one study, subjects who had smoked marijuana experienced unusual difficulty in transferring information from short- to long-term memory. They could recall items for a brief period, but as time progressed, they forgot more and more of the information(Klatzky, 1980). Like alcohol, marijuana appears to interfere with the ability to encode and store data in long-term memory. There are drugs that actually enhance memory. Perhaps the most promising of these is DDAVP, a synthetic drug related to the hormone vasopressin, which is secreted by the pituitary gland. Vasopressin appears to increase a person" s motivation and enjoyment of learning, and these are important factors in improving one" s memory. Taken in the form of a nasal spray, DDAVP has significantly increased subjects" ability to recall information on cue. Another hormone, epinephrine, may also aid memory. Gold and Delaney(1981)discovered that the memory-enhancing effects of epinephrine were greatest when it was administered right after a learning session. Delays in administering the epinephrine markedly reduced its effectiveness. These results suggest that naturally occurring memory problems may be due in part to inadequate levels of vasopressin or epinephrine(McGaugh, 1983).