单项选择题

I'm sure the letter won't arrive in time（）it is sent by air.

A．if
B．until
C．because
D．unless

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单项选择题

Children shouldn't（）on the conversation between their parents and others.

A．break in
B．break down
C．break out
D．break away.

单项选择题

（）man must fear when traveling in space is radiation(辐射) from the sun.

A．Which
B．How
C．What
D．That

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