单项选择题

No one hopes that he will ______ his usefulness.

A．retain
B．keep
C．prolong
D．survive
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单项选择题
Hardly ______ when a loud explosion was heard.
A．the train had started
B．the train had been started
C．had the train started
D．did the train start
单项选择题
The research institute has recruited 25 university graduates ______ in science.
A．majored
B．majoring
C．having majored
D．have majored
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