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
The word subjects (first sentence, last paragraph) refers to ______.
A．the performance tests used in the study of sleep deficit
B．special branches of knowledge that are being studied
C．people whose behaviors or reactions are being studied
D．the psychological consequences of sleep deficit
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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