题目内容：
Decide which of the choices given below would best complete the passage if inserted in the corresponding blanks. Mark the best choice for each blank on your answer sheet.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 (31)______ the hours of darkness. Our present (32) ______ is with how easily and to what extent this (33) ______ can be modified.
The question is no mere academic one. The ease (34) ______ people can change from working in the day to working at night is a (35) ______of growing importance in industry where automation (36) ______ round-the-clock working of machines. It normally (37) ______ from five days to one week for a person to (38)______ to a(39)______ routine of sleep and wakefulness, sleeping during the day and working at night. (40) ______, 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 (41) ______ he has to change to another,(42)______ much of his time is spent neither working nor sleeping very (43)
One answer would seem to be (44) ______ periods on each shift, a month, or even three months. (45) _______, recent research has shown that people on such systems will revert to go back to their (46) ______ habits of sleep and wakefulness during the weekend and that this is quite enough to destroy any (47) ______ to night work built up during the week. The only real solution appears to be to (48) ______ the night shift to those (49) ______ night workers whose (50)______ may persist through weekends and holidays._________填上最佳答案。 A．in
B．with
C．of
D．over
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