单项选择题

You can' t ______your promise; all the people here are depending on you.

A．back off
B．back up
C．back on to
D．back out of
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单项选择题
Fiber-optic cables can carry hundreds of telephone conversations ______.
A．simultaneously
B．spontaneously
C．homogeneously
D．contemporarily
单项选择题
The husband' s story and his wife' s story ______each other.
A．retract
B．contract
C．distract
D．contradict
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