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
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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
单项选择题
Mary ______ in the garden when it began to rain.
A．was walking
B．walked
C．walking
D．had walked
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