单项选择题

Sleep is something we generally associate with living creatures. Of course, it is (36) that a lot of animals sleep, but zoologists are certain that (37) forms of animal life, (38) worms and snails, never really sleep. On the other (39) , animals such as bears sleep for 4 (40) 5 months every year.
The (41) of sleep a human being needs (42) on age, the individual and possible (43) . For example, doctors think that pre school children need between 10 and 12 hours a night, school children between 9 and 11 hours, and (44) between 7 and 8 hours. There are exceptional (45) of old people (46) only sleep between 2 and 3 hours a day and (47) to be active and healthy. The sleep (48) of different (49) also appear to be (50) . Japanese people for example, sleep (51) hours than Europeans.
(52) is not known for (53) whether mental activity (54) when a person is asleep. However, it is certainly true that some people can wake (55) at a specific, pre-determined time.

49().

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