单项选择题

I first met Mary three years ago when we ______in a middle school together.

A．have worked
B．had been working
C．were working
D．had worked
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单项选择题
______ you understand this rule, you'll have no further difficulty.
A．Once
B．Unless
C．Only
D．Until
单项选择题
He found it necessary for us______ from each other.
A．to help and learn
B．helping and learning
C．to learn and to help
D．learning and helping
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