[单选题]
21. No one knows why we sleep, but it’s certain that we need to. People who are prevented from sleeping begin to suffer obvious effect after a few days --- they think less clearly, and they fall asleep during the working hours. Some may have hallucinations (幻觉).
There are no rules about sleep. Generally speaking, grown-ups sleep about seven and a half hours each night and probably more than 60 percent get between seven and eight hours. But perhaps eight percent are quite happy with five hours or less, and four percent or so find that they want ten hours or more. If you feel all right, you’re probably getting enough sleep. The important thing is not to worry how much other people get --- their needs may be different. Exercise doesn’t seem to increase the need for sleep, office workers, for example, sleep for about as long as people doing physically active work.
Children sleep more than grown-ups --- perhaps fourteen to eighteen hours soon after birth, going down to grown-up levels by early teen-age. Sleep patterns also tend to be different in the elderly, who may sleep less at night than they did when younger. They find sleep getting more broken, and often make it a rule to sleep during the daytime.(10分)
21.1 According to the passage, some people are unable to think clearly because ________.(2分)
A. they have hallucinations
B. they sleep for too many hours
C. they don’t have enough sleep
D. they feel sleepy during the working hours
A
B
C
D
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