单项选择题

______ several times, they need some encouragement.

A．Being failed
B．Having failed
C．To fail
D．To be failed
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单项选择题
A man escaped from the prison last night. It was a long time ______ the guards discovered what had happened.
A．before
B．until
C．since
D．when
单项选择题
The tourist is forbidden to enter a country if he does not have a (an) ______ passport.
A．operative
B．valid
C．effective
D．efficient
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