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
The word radical in the third paragraph is closest in meaning to ______.
A．equal
B．extreme
C．excellent
D．basic
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.
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