单项选择题

Crops grew smaller in production in South Carolina mainly because ______.

A．demand for cotton had decreased
B．crops became varied
C．planters grew richer
D．soil was less productive
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单项选择题
The main topic of the passage is ______.
A．laughter and physical exercise have similar effects on the human body
B．smile can produce the same effects as laughter
C．pain can be reduced by laughter
D．laughter is the best medicine
单项选择题
It has been proved all of the following except that ______.
A．smiling does good to health
B．laughter can produce pain
C．there is a way to reduce pain
D．laughter can work the muscles in the feet
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