单项选择题

______ you return those books to the library immediately, you will have to pay a fine.

A．Until
B．If
C．Unless
D．Provided
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单项选择题
Prisons in some countries are short of staff, ______ means each prison officer is overworked and underpaid.
A．which
B．this
C．what
D．it
单项选择题
Had he been given some information, he ______ the questions.
A．answered
B．could have answered
C．answers
D．was answering
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