2. 外语
  3. The amount of sleep a human being needs depends on【C1】______ , the individual and possibly race. For example, doctors think that

The amount of sleep a human being needs depends on【C1】______ , the individual and possibly race. For example, doctors think that

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问题     The amount of sleep a human being needs depends on【C1】______ , the individual and possibly race. For example, doctors think that pre-school children need between 10 and 12 hours a night; school children between 9 and 11 hours; and adults between 7 and 9 hours. There are exceptional【C2】______ of old people who only sleep between 2 and 3 hours a day and continue to be active and healthy. The sleep requirements of different【C3】______ also appear to be different. Japanese people, for example, sleep fewer hours than Europeans.
    It is not known for【C4】______ if mental activity(apart from dreaming)occurs when a person is asleep.【C5】______ , it is certainly true that some people can wake up at a specific, predetermined time. There are also【C6】______ about mathematicians who solve difficult problems during sleep, because their【C7】______ minds continue working on the problem.
    Some types of unpleasant dreams(or "nightmares")are quite【C8】______ ; the dreamer is taking a test, but is not properly prepared; he is falling from a tree;【C9】______ an animal or thing is chasing him. Some people say these last two occur【C10】______ man’s ancestors lived in trees and were in constant danger from wild animals.
【C2】
选项 A、causes
B、cases
C、factors
D、proofs
答案B
解析 A项causes意为："原因"，B项cases意为："情况"，C项factors意为："因素，要素"，D项proofs意为："证据，证明"。从句意看，空格中应填入B项。本句的意思是："有一天只睡2至3小时，却依然非常活跃、健康的老人的例外情况"。其他三个选项均与句意不符。
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