单项选择题

The doctor will not perform the operation () it is absolutely necessary.

A. so
B. if
C. for
D. unless

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单项选择题
Hardly had he finished his speech () the audience started cheering.

A. then
B. when
C. than
D. as
单项选择题
In some parks visitors are requested to keep () the grass.

A. aside
B. away
C. out
D. off
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