单项选择题

The author' s purpose in writing this article is to ______.

A．inform readers of Heller' s findings in his researches
B．persuade readers to get enough sleep at different ages
C．show readers the importance of getting enough sleep
D．tell readers how many hours of sleep they should get
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单项选择题
The phrase such powerful effects in Line 3 of Paragraph 3 refers to ______.
A．the fact that many people cannot sleep and wake refreshed
B．the fact that the brain puts us to sleep when we are worn out
C．your sight, hearing, and attention are slow to alert you to dangers
D．increased risk of health problems or a serious, even deadly accident
单项选择题
It can be inferred from the passage that ______.
A．people of an older age are more likely to suffer from deficient sleep
B．the older one becomes, the more sleep he or she will need every night
C．everyone of us needs to have seven or eight hours of sleep every night
D．even if we are tired, we can still react quickly as soon as we find dangers
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