单项选择题

These machines must ______ before they are used.

A． check
B． be checked
C． are checked
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单项选择题
The horse is getting old and cannot run ______ it did.
A． as fast as
B． as faster as
C． as faster than
单项选择题
Don't keep the boy ______ outside. It's too cold (寒冷的,伤风) there.
A． waits
B． to wait
C． waiting
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