单选题The last paragraph suggests

单选题My own inclination, if I were in your situation, would be to look for another position. A. symptom B. likeness C. habit D. tendency

单选题One of the ______ of the training program is that it enables the young people to be better candidates for employment.

单选题Which has been done by World Health Organization in combating the bad effects of influenza?

单选题I personally am offended by what they have tried to do in a very misleading way with, what I've said about two of my personal ______, President Kennedy and Dr. Martin Luther King Jr. A. resemblance B. statues C. icons D. parable

单选题The president gave a ______ for the visiting heads of state.

单选题In the past few decades, remarkable findings have been made in ethology, the study of animal social behavior. Earlier scientists had【C1】______that nonhuman social life was almost totally instinctive or fixed by genetics. Much more careful observation has shown that【C2】______variation occurs among the social ties of most species, showing that learning is a part of social life. That is, the【C3】______are not solely fixed by the genes.【C4】______, the learning that occurs is often at an early age in a process that is called imprinting. Imprinting is clearly【C5】______instinctive, but it is not quite like the learning of humans; it is something in between the two. An illustration best【C6】______the nature of imprinting. Once, biologists thought that ducklings followed the mother duck because of instincts. Now we know that, shortly【C7】______they hatch, ducklings fix【C8】______any object about the size of a duck and will henceforth follow it. So ducklings may follow a basketball or a briefcase if these are【C9】______for the mother duck at the time when imprinting occurs. Thus, social ties can be considerably【C10】______, even ones that have a considerable base【C11】______by genetics. Even among the social insects something like imprinting【C12】______influence social behavior. For example, biologists once thought bees communicated with others purely 【C13】______instinct. But, in examining a "dance" that bees do to indicate the distance and direction of a pollen source, observers found that bees raised in isolation could not communicate effectively. At a higher level, the genetic base seems to be much more for an all-purpose learning rather than the more specific responses of imprinting. Chimpanzees, for instance, generally【C14】______very good mother but Jane Goodall reports that some chimps carry the infant upside down or【C15】______fail to nurture the young.

单选题The day before my history exam, I still hadn't ______ reading the first book on the list. [A] seen about [B] caught up with [C] got around to [D] sat for

单选题It is only with further evolution and refinement that health plan report cards can ______ their potential and become a distinctive and useful tool.

单选题The government decided to take a______action to strengthen the market management.(清华大学2007年试题)

单选题Much of the research on hallucinogenic drugs such as LSD has focused on the neurotransmitter serotonin, a chemical that when released from a presynaptic serotonin-secreting neuron causes the transmission of a nerve impulse across a synapse to an adjacent postsynaptic, or target, neuron. There are two major reasons for this emphasis. First, it was discovered early on that many of the major hallucinogens have a molecular structure similar to that of serotonin. In addition, animal studies of brain neurochemistry following administration of hallucinogens invariably reported changes in serotonin levels. Early investigators correctly reasoned that the structural similarity to the serotonin molecule might imply that LSD' s effects arc brought about by an action on the neurotransmission of serotonin in the brain. Unfortunately, the level of technical expertise in the field of brain research was such that this hypothesis had to be tested or, peripheral tissue ( tissue outside the brain). Two different groups of scientists reported than LSD powerfully blockaded serotonin' s action, their conclusions were quickly challenged, however. We now know that the action of a drug at one site in the body does not necessarily correspond to the drug' s action at another site, especially when one site is in the brain and the other is not. By the 1960' s technical advances permitted the direct testing of the hypothesis that LSD and related hallucinogens act by directly suppressing the activity of serotonin secreting neurons themselves—the so called presynaptic hypothesis. Researchers reasoned that if the hallucinogenic drags act by suppressing the activity of serotonin-secreting neurons, then drugs administered after these neurons had been destroyed should have no effect on behavior, because the system would already be maximally suppressed. Contrary to their expectations, neuron destruction enhanced the effect of LSD and related hallucinogens on behavior. Thus hallucinogenic drugs apparently do not act directly on serotonin-secreting neurons. However, these and other available data do support an alternative hypothesis, that LSD and related drugs act directly at receptor sites on serotonin target neurons (the postsynaptic hypothesis). The fact that LSD elicits "serotonin syndrome", that is, causes the same kinds of behaviors as does the administration of serotonin in animals whose brains are depleted of serotonin indicates that LSD acts directly on serotonin receptors, rather than indirectly through the release of stores of serotonin. The enhanced effect of LSD reported after serotonin depletion could be due to a proliferation of serotonin receptor sites on serotonin target neurons. This phenomenon often follows neuron destruction or neurotransmitter depletion; the increase in the number of receptor sites appears to be a compensatory response to decreased input. Significantly, this hypothesis is supported by number of different laboratories,

单选题1 All animals must rest, but do they really sleep as we know it? The answer to this question seems obvious. If an animal regularly stops its activities and stays quiet and un moving-if it looks as though it is sleeping—then why not simply assume that it is in fact sleeping? But how can observers be sure that an animal is sleeping? They can watch the animal and notice whether its eyes are open or closed, whether it is active or lying quietly, and whether it responds to light or sound. These actors are im portant clues, but they often are not enough. Horses and cows, for example, rarely close their eyes, and fish and snakes cannot close them. Yet this does not necessarily mean that they do not sleep. Have you ever seen a cat dozing with an eye partly open? Even humans have occasionally been observed to sleep with one or both eyes partially open. Animals do not necessarily lie down to sleep either. Elephants, for example, often sleep standing up, with their tusks resting in the fork of a tree. Finally, while "sleeping" animals often seem unaware of changes in the sounds and light and other stimuli around them, that does not really prove they are sleeping either. Observations of animal behavior alone cannot fully answer the question of whether or not animals sleep. The answers come from doing experiments in sleep laboratories, using a machine called the electroencephalograph (EEC). The machine is connected to animals and measures their brain signals, breathing, heartbeat, and muscle activity. The measure- ments are dierent when the animals appear to be sleeping than when they appear to be awake. Using the EEC, scientists have confirmed that all birds and mammals studied in laboratories do sleep. There is some evidence that reptiles, such as snakes and turtles, do not truly sleep, although they do have periods of rest each day, in which they are quiet and unmoving. They also have discovered that some animals, like chimpanzees, cats, and moles (who live underground), are good sleepers while others, like sheep, goats, and donkeys, are poor sleepers. Interestingly, the good sleepers are nearly all hunters with rest ing places that are safe from their enemies. Nearly all the poor sleepers are animals hunted by other animals: they must always be watching for enemies, even when they are resting.

单选题The manager threw a party______the group of computer experts from the United States.

单选题In view of the depression of the national economy, there are plans to ______, a much greater harvest from the timber resource.

单选题With tears in her eyes, the champion was {{U}}oblivious{{/U}} to the cheering in the stands.

单选题A lie is as much a lie, when it is whispered, as when it is______at the market cross.

单选题He goes shopping so frequently not because he is rich, but because he enjoys ______ politely.

单选题With her last child having left home, she felt a ______ need to fill her time.