题目内容：
Directions: There are 20 blanks in the following passage. For each blank there are four choices marked A, B, C and D on the right side of the paper. You should choose the ONE that best fits into the passage. Then mark the corresponding letter on Answer Sheet 2 with a single line through the centre.The normal human daily cycle of activity is of some 7- 8 hours. Sleep alternation with some 16-17 hours' 1 and that the sleep normally coincides 2 the hours of darkness. Our present 3 is how easily and to what extent this 4 can be modified.
The question is no mere academic one. The case with which people can change from working in the day to working at night is a 5 of growing importance in industry where automation 6 for round the clock working of machines.It normally 7 from five days to one week for a person to adapt to a 8 routine of sleep and wakefulness, sleeping during the day and working at night. 9 , it is often the case in industry that 10 are changed every week. This means that no sooner has he got used to one routine 11 he has to change to another, 12 that much of his time is spent 13 working nor sleeping very 14 . One answer would seem to be 15 periods on each shift, a month, or even three months. 16 , recent research has shown that people on such systems will revert to go back to their 17 habits of sleep and wakefulness during the weekend and that this is quite enough to destroy any 18 to night work built up during the week. The only real solution appears to be to hand 19 the night shift to those permanent night workers whose 20 may persist through all weekends and holidays. A．weakness
B．fairness
C．wakefulness
D．goodness
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