2. 外语
  3. The normal human daily cycle of activity is of some 7-8 hours’ sleep alternation with some 16-17 hours’ wakefulness and that the

The normal human daily cycle of activity is of some 7-8 hours’ sleep alternation with some 16-17 hours’ wakefulness and that the

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问题     The normal human daily cycle of activity is of some 7-8 hours’ sleep alternation with some 16-17 hours’ wakefulness and that the sleep normally coincides【C1】______the hours of darkness. Our present concern is with how easily and to what extent this【C2】______can be modified.
    The question is no mere academic one. The ease with which people can change from working in the day to working at night is a【C3】______of growing importance in industry where automation【C4】______round-the-clock working of machines. It normally【C5】______form five days to one week for a person to adapt to a【C6】______routine of sleep and wakefulness, sleeping during the day and working at night. 【C7】______, it is often the case in industry that shifts are changed every week. This means that no sooner has he got used to one routine【C8】______he has to change to another, 【C9】______much of his time is spent neither working nor sleeping every 【C10】______. One answer would seem to be【C11】______periods on each shift, a month, or even three months. 【C12】______recent research has shown that people on such systems will revert to go back to their【C13】______habits to sleep and wakefulness during the week-end and that this is quite enough to destroy any【C14】______to night work built up during the week. The only real solution appears to be to hand over the night shift to those permanent night workers whose【C15】______may persist through all weekends and holidays.  
【C2】
选项 A、cycle
B、period
C、circle
D、round
答案A
解析 此处应选择A，根据句子中的指示代词this可知，其后所接单词应该是前面提到过的事物，根据句意判断，应该是cycle。
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