单项选择题

Trees take harmful gas from the air, and ______ oxygen (氧气)into the air.

A． let out
B． set off
C． give up
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Use an umbrella to protect yourself ______ the rain.
A． of
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C． from
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Ice and snow ______ houses near the North pole.
A． used to build
B． are used to building
C． are used to build
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