根据下列材料，请回答下列各题 We all know that the normal human daily cycle of activity is

of some 7-8 hours sleep alternating with some16-17 hours wakefulness and that, broadly speaking, the sleep normally coincides with the hours of darkness. Ourpresent concern is with how easily and to what extent this cycle can be modified. The question is no mere academic one. The ease, for example, with which people can change from working inthe day to working at night is a question of growing importance in industry where automation calls for round-the-clock working of machines. It normally takes from five days to one week for a person to adapt to a reversed routine ofsleep and wakefulness, sleeping during the day and working at night. Unfortunately, it is often the case in industrythat shifts are changed every week; a person may work from 12 midnight to 8 a.m. one week, 8 a.m. to 4 p.m. the next, and 4 p.m. to 12 midnight the third and so on. This means that no sooner has he got used to one routine than he has tochange to another, so that much of his time is spent neither working nor sleeping very efficiently. One answer would seem to be longer periods on each shift, a month, or even three months. Recent research byBonjer of the Netherlands, however, has shown that people on such systems will revert to their normal habits ofsleep and wakefulness during the weekend and that this is quite enough to destroy any adaptation to night workbuilt up during the week. The only real solution appears to be to hand over the night shift to a number of permanent night workers. Aninteresting study of the domestic life and health of night-shift workers was carried out by Brown in 1957. Shefound a high incidence of disturbed sleep and other disorders among those on alternating day and night shifts, butno abnormal occurrence of these phenomena among those on permanent night work. This latter system then appears to be the best long-term policy, but meanwhile something may be done torelieve the strains of alternate day and night work by selecting those people who can adapt most quickly to thechanges of routine. One way of knowing when a person has adapted is by measuring his body temperature. Peopleengaged in normal day-time work will have a high temperature during the hours of wakefulness and a low one atnight; when they change to night work the pattern will only gradually go back to match the new routine and thespeed with which it does so parallels, broadly speaking, the adaptation of the body as a whole, particularly in termsof performance. Therefore, by taking body temperature at intervals of two hours throughout the period ofwakefulness it can be seen how quickly a person can adapt to a reversed routine, and this could be used as a basisfor selection. So far, however, such a form. of selection does not seem to have been applied in practice. The study of sleep cycle can be of practical use to places where________

A. strict work shifts are required to cater for the need of automatic machines

B. working at night is normal for most employees

C. employees work according to different requirements of machines

D. employees have to adapt to irregular work patterns and organizations