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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单项选择题
Mary ______ in the garden when it began to rain.
A．was walking
B．walked
C．walking
D．had walked
单项选择题
______ in the air, fuels give off heat.
A．To burn
B．Burned
C．To be burned
D．Being burned
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