单选题Walking to Exercise the Brain Do you think sitting and studying all the time will improve your grades? Think again. Getting some exercise may help, too. New research with older people suggests that taking regular walks helps them pay attention better than if they didn't exercise. Previous research had shown that mice learn, remember, and pay attention better after a few weeks of working out on a running wheel. Mice that exercise have greater blood flow to the brain than those who don't. Their brain cells also make more connections. Neuroscientists from the University at Urbana-Champaign wanted to find out if the same thing is true for people. First, they measured the physical fitness of 41 adults, ages 58 to 77, after each person walked 1 mile. Then, participants looked at arrows on a computer screen and had to use computer keys to show which way one particular arrow was pointing. Adults who were physically fit were faster at the arrow task, and their answers were just as accurate as their less fit peers, the researchers found. The fitter participants also had more blood flow to a part of their brain responsible for paying attention and making decisions. In a second study, 15 elderly people who completed a 6-month aerobic-training course were faster at attention tasks compared with 14 seniors who just did stretching and toning exercises for the same amount of time. So, even going for a walk every 2 or 3 days for just 10 to 45 minutes can help. That should be good news for your grandparents. The effects of exercising on the brains of younger people haven't been studied yet. Still, it can't hurt to take occasional study breaks and go for a walk or run around with your friends. You might even do better in school. Whatever you do, though, don't try to read and walk at the same time. You could end up hurting yourself!

单选题Where Has the Salt Come from? Every now and then, we meet a fact about our earth that makes us feel strange and no answer for the fact has yet been found. Such a fact is the existence of salt in the oceans. How did it get there? We simply do not know how the salt got into the ocean! We do know, of course, that salt is water-soluble, and so passes into the oceans with rainwater. The salt of the earth's surface is constantly being dissolved (溶解) and is passing into the ocean. But we do not know whether this can explain the huge quantity of salt in oceans. If all the oceans were dried up, enough salt would be left to build a wall 180 miles high and a mile thick. Such a wall would reach once around the world at the Equator (赤道)! The common salt that we all use is produced from seawater or the water of salt lakes, from salt springs (源泉) and from deposits of rock salt. The concentration (浓度) of salt in seawater ranges from about three percent to three-and-one-half percent. The Dead Sea, which covers an area of about 340 square miles, contains about 11,600,000,000 tons of salt! On the average, a gallon (加仑) of seawater contains about a quarter of a pound of salt. The beds of rock salt that are found in various parts of the world were all originally formed by the evaporation (蒸发) of seawater millions of years ago. It is believed that the thick rock-salt deposits were formed after about nine-tenth of the volume of seawater had been evaporated. Most commercial salt is obtained from rock salt. The usual method is to drill wells (井) down to the salt beds. Pure water is pumped down (抽进去) through a pipe. The water dissolves the salt and it is forced through another pipe up to the surface.

单选题I don't quite follow what she is saying. A. believe B. understand C. explain D. accept

单选题Guests were scared when the bomb exploded.

单选题The researchers have come up with numerous explanations to ______ their failures.A. excuseB. justifyC. admitD. avoid

单选题Biologists have ascertained that specialized cells convert chemical energy into mechanical energy.

单选题The Development of American Postal System In the early days of the United States, postal charges were paid by the recipient, and charges varied with the distance carried. In 1825, the United States Congress permitted local postmasters to give letters to mail carriers for home delivery, but these carriers received no government salary and their entire compensation depended on what they were paid by the recipients of individual letters. In 1847, the United States Post Office Department adopted the idea of a postage stamp, which of course simplified the payment for postal service but caused grumbling by those who did not like to prepay. Besides, the stamp covered only delivery to the post office and did not include carrying it to a private address. In Philadelphia, for example, with a population of 150,000, people still had to go to the post office to get their mail. The confusion and congestion of individual citizens looking for their letters was itself enough to discourage use of the mail. It is no wonder that, during the years of these cumbersome arrangements, private letter-carrying and express businesses developed. Although their activities were only semi-legal, they thrived, and actually advertised that between Boston and Philadelphia they were half-day speedier than the government mail. The government postal service lost volume to private competition and was not able to handle efficiently even the business it had. Finally, in 1863, Congress provided that the mail carriers who delivered the mail from the post offices to private addresses should receive a government salary, and that there should be no extra charge for that delivery. But this delivery service was at first confined to cities, and free home delivery became a mark of urbanism. In 1890, of the 75 million people in the United States, fewer than 20 million had mail delivered free to their doors. The rest, nearly three quarters of the population, still received no mail unless they went to their post office.

单选题These factors interact {{U}}intimately{{/U}} and cannot be separated.

单选题This kind of material was {{U}}seldom{{/U}} used in building houses during the middle ages.

单选题Plant Gas Scientists have been studying natural sources of methane (甲烷，沼气) for decades but hadn't regarded plants as a producer, notes Frank Keppler, a geochemist(地球化学家) at the Max Planek Institute for Nuclear Physics in Heidelberg, Germany. Now Keppler and his colleagues find that plants, from grasses to trees, may also be sources of the greenhouse gas. This is really surprising, because most scientists assumed that methane production requires an oxygen-free environment. Previously, researchers had thought that it was impossible for plants to make significant amounts of the gas. They had assumed that microbes(微生物) need to be in environments without oxygen to produce methane. Methane is a greenhouse gas, like carbon dioxide. Gases such as methane and carbon dioxide trap heat in Earth's atmosphere and contribute to global warming. In its experiments, Keppler's team used sealed chambers (室，房间；腔) that contained the same concentration of oxygen that Earth's atmosphere has. They measured the amounts of methane that were released by both living plants and dried plant material, such as fallen leaves. With the dried plants, the researchers took measurement at temperatures ranging from 30 degrees Celsius to 70 degrees C..At 30 degrees C., they found, a gram of dried plant material released up to 3 nanograms(微克) of methane per hour (One nanogram is a billionth of a gram. ). With every 10-degree rise in temperature, the amount of methane released each hour roughly doubled. Living plants growing at their normal temperatures released as much as 370 nanograms of methane per gram of plant tissue per hour. Methane emissions tripled (增加三倍) when living and dead plant was exposed to sunlight. Because there was plenty of oxygen available, it's unlikely that the types of bacteria( bacterium 的复数，细菌) that normally make methane were involved. Experiments on plants that were grown in water rather than soil also resulted in methane emissions. That's another strong sign that the gas came from the plants and not soil microbes. The new finding is an "interesting observation," says Jennifer Y.King, a biogeochemist(生物地球化学家) at the University of Minnesota in St. Paul. Because some types of soil microbes consume methane, they may prevent plant-produced methane from reaching the atmosphere. Field tests will be needed to assess the plant's influence, she notes.

单选题Unfortunately war {{U}}started{{/U}} suddenly in the Middle East on October 6, 1973.

单选题This article requires more attention to the problem of cultural interferenceA. calls forB. applies forC. cares forD. allows for

单选题As a good student, you have to follow the school rules.

单选题Although the working mother is very busy she still devotes a lot of time to her children.

单选题When these occur, we tend to Uresort/U to background tendencies and habits to justify our behavior.

单选题The Development of American Postal System In the early days of the United States, postal charges were paid by the recipient, and charges varied with the distance carried. In 1825, the United States Congress permitted local postmasters to give letters to mail carriers for home delivery, but these carriers received no government salary and their entire compensation depended on what they were paid by the recipients of individual letters. In 1847, the United States Post Office Department adopted the idea of a postage stamp, which of course simplified the payment for postal service but caused grumbling by those who did not like to prepay. Besides, the stamp covered only delivery to the post office and did not include carrying it to a private address. In Philadelphia, for example, with a population of 150,000, people still had to go to the post office to get their mail. The confusion and congestion of individual citizens looking for their letters was itself enough to discourage use of the mail. It is no wonder that, during the years of these cumbersome arrangements, private letter-carrying and express businesses developed. Although their activities were only semi-legal, they thrived, and actually advertised that between Boston and Philadelphia they were half-day speedier than the government mail. The government postal service lost volume to private competition and was not able to handle efficiently even the business it had. Finally, in 1863, Congress provided that the mail carriers who delivered the mail from the post offices to private addresses should receive a government salary, and that there should be no extra charge for that delivery. But this delivery service was at first confined to cities, and free home delivery became a mark of urbanism. In 1890, of the 75 million people in the United States, fewer than 20 million had mail delivered free to their doors. The rest, nearly three quarters of the population, still received no mail unless they went to their post office.

单选题The authorities have drawn up Uan elaborate/U plan to combat increasing crime in the city.

单选题When did you first {{U}}encounter{{/U}} these difficulties?

单选题They promote {{U}}assimilation{{/U}} of ethnic groups into the main-stream culture.

单选题Longer Lives for Wild Elephants Most people think of zoos as safe places for animals, where struggles such as having difficulty finding food and avoiding predators (猛兽) don't exist. Without such problems, animals in zoos should live to a ripe (成熟的) old age. But that may not be true for the largest land animals on Earth. Scientists have known that elephants in zoos often suffer from poor health. Sometimes, they even become unable to have babies. To learn more about how captivity (圈养) affects elephants, a team of international scientists compared the life spans of female elephants born in zoos with female elephants living outdoors in their native lands. Zoos keep detailed records of all the animals in their care, documenting factors such as birth dates, illnesses, weight and death. These records made it possible for the researchers to analyze 40 years of data on 800 African and Asian elephants in zoos across Europe. The scientists compared the life spans of the zoo-born female elephants with the life spans of thousands of wild female elephants in Africa and Asian elephants that work in logging camps (伐木场), over approximately the same time period. The team found that female African elephants born in zoos lived an average of 16.9 years. Their wild counterparts who died of natural causes lived an average of 56 years--more than three times as long. Female Asian elephants followed a similar pattern. In zoos, they lived 18.9 years, while those in the logging camps lived 41.7 years. Scientists don't know yet why wild elephants seem to get on so much better than their zoo-raised counterparts. Georgia Mason, a biologist at the University of Guelph in Canada who led the study, thinks stress and obesity (肥胖症) may be to blame. Zoo elephants don't get the same kind of exercise they would in the wild, and most are very fat. Social lives of elephants are also much different in zoos than in the wild, where they live in large herds and family groups. The study raises some questions about acquiring more elephants to keep in zoos. While some threatened and endangered species living in zoos reproduce (生殖) successfully and maintain healthy populations, that doesn't appear to be the case with elephants.