单项选择题

After how many minutes does an average person repeat the sleep cycle

A．8 to 15 minutes.
B．30 minutes.
C．40 to 60 minutes.
D．80 minutes.
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单项选择题
Which of the following does the passage mainly discuss
A．Scientific methods of studying sleep.
B．Why humans require sleep.
C．The phases of sleep.
D．Why dreams occur during REM sleep.
单项选择题
According to the passage, the alpha rhythm is the characteristic of which phase of sleep
A．Stage 1.
B．Stage 2.
C．Stage 3.
D．Stage 4.
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