单项选择题

Some cities have passed laws that allow coal and oil______only if their sulfur content is low.

A．burning
B．to burn
C．being burned
D．to be burned
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单项选择题
It is time to______fields in which they are just as capable as men.
A．keep women's
B．stop to keep women out
C．keep women away
D．stop keeping women out of
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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