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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单项选择题

Many Americans believe that ______.

A．sleep is the first thing that can be sacrificed when one is busy
B．they need more sleep to cope with the complexities of everyday life
C．to sleep is something one can do at any time of the day
D．enough sleep promotes people's drive and ambition

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