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
Preparations are being made for the Olympic Games ______ in Beijing in 2008.
A．held
B．holding
C．to hold
D．to be held
单项选择题
If ______ to a high temperature, ice will change to water.
A．heating
B．being heated
C．to heat
D．heated
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