单项选择题

According to this passage, sleep is described as __________.

A．drowning in an ocean of darkness
B．undisturbed by tension or problems
C．a gradual parting from the real world
D．dependent only upon individual age and health

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单项选择题

According to the passage, what was used by sleep researchers

A．Music.
B．Ladder.
C．Charts.
D．Signature.

单项选择题

This passage states that a person is really asleep only __________.

A．after experiencing a jerk
B．when his EEG reveals no alpha rhythm
C．after the completion of his alpha rhythm
D．when his EEG begins to show an alpha rhythm