单项选择题

When the mixture ______, it will give off a powerful force.

A．will heat
B．will be heated
C．is heated
D．has heated
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单项选择题
If a better material______, the strength of the part would have been in- creased.
A．had been used
B．had been using
C．being used
D．using
单项选择题
It is necessary that we ______ have a good command of English at col- lege.
A．must
B．have to
C．should
D．ought to
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