单项选择题

Once environmental damage ______, it takes many years for the system to recover.

A．is done
B．has done
C．will be done
D．is to be done
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单项选择题
Have your files protected when working on the computer, ______ you might have them deleted by accident.
A．while
B．so
C．and
D．or
单项选择题
It was so deep in the night. All was quiet ______ an occasional baby cry in the neighboring building.
A．except for
B．but
C．except
D．beside
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