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
Beijing is well______its beautiful scenery and the Great Wall.
A．known as
B．known to
C．known about
D．known for
单项选择题
It was not until it got dark______working.
A．that they stopped
B．when they stopped
C．did they stop
D．that they didn't stop
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