单项选择题

A．The purpose of laughter.
B．The cause of laughter and its effects.
C．Who and when people laugh.
D．The origins of laughter.
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单项选择题
Sleep is described as ______.
A．a gradual parting from the real world
B．undisturbed by tension or problems
C．drowning in an ocean of darkness
D．dependent only Upon individual age and health
单项选择题
The term subjects (line 7, para. 3) refers to ______.
A．waves
B．dreams
C．volunteers
D．instruments
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