单项选择题

People in the 18th and 19th centuries used to sleep about 9. 5 hours a night because they had ______.

A．no drive and ambition
B．no electric lighting
C．the best sleep habits
D．nothing to do in the evening
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单项选择题
It can be concluded from the passage that one should sleep as many hours as is necessary to ______.
A．improve one's memory dramatically
B．be considered dynamic by other people
C．maintain one's daily schedule
D．feel energetic and perform adequately
单项选择题
According to Dr. David, Americans ______.
A．are ideally vigorous even under the pressure of life
B．often neglect the consequences of sleep deficit
C．do not know how to relax themselves properly
D．can get by on 6.5 hours of sleep
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