单项选择题

Whenever work is being done, energy（）from one form into another.

A. converts
B. converted
C. is converted
D. is being converted

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单项选择题
A light sleeper is usually very （）to any sound even as inaudible as the humming of a mosquito.

A. acute
B. keen
C. alert
D. immune
单项选择题
Although most universities in the United States are on a semester system which offers classes in the fall and spring, some schools（）a quarter system comprised of fall, winter, and summer quarters.

A. manipulate
B. stipulate
C. regulate
D. observe
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