单项选择题

Air is the mixture of gases in which () the gas is oxyen.

A．not every of
B．not all
C．none of
D．all of

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单项选择题
I'm sorry I'm late, I was held up in a traffic ().

A．block
B．group
C．jam
D．bunch
单项选择题
—She was in class on Friday afternoon, but no one has seen her since.—She () an accident.

A．might have
B．might have had
C．may have
D．ought to have had
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