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  3. In the past few decades, remarkable findings have been made in ethology, the study of animal social behavior. Earlier scientists

In the past few decades, remarkable findings have been made in ethology, the study of animal social behavior. Earlier scientists

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问题     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. She believes that these females were the youngest or the 【C16】______ child of a mother. In such circumstances, they did not have the opportunity to observe how their own mother【C17】______for her young. Certainly adolescent chimps who are still with their mothers when other young are born take much interest in the rearing of their young brother or sister. They have an excellent opportunity to learn, and the social ties that are created between mother and young 【C18】______ Goodall to describe the social unit as a family. The mother offspring tie is beyond 【C19】______; there is some evidence to 【C20】______ that ties also continue between siblings of the same sex, that is "brother-brother" and "sister-sister".
【C12】
选项 A、may
B、should
C、must
D、can
答案C
解析 考查情态动词。由蜜蜂的例子可以看出，甚至在昆虫之中，类似“印象”的东西也必然影响着其他社会行为，may，should，can的语气均不如must强。故C．must为正确答案。
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