单项选择题

The stages of sleep take on______.

A．an irregular aspect
B．a regular aspect
C．a punctual aspect
D．a similar aspect
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单项选择题
The brain waves are the slowest during______.
A．stage 1
B．stage 2 and stage 3
C．stage 4
D．REM sleep
单项选择题
Stage 4 sleep lasts______.
A．about 80 minutes
B．about 40-60 minutes
C．about 30 minutes
D．about 2040 minutes
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