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.
【C12】

选项 A、may
B、should
C、must
D、can

答案C

解析 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。
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