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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单项选择题
John is always ready ______ others.
A．helps
B．to help
C．help
单项选择题
My classmate asked me ______.
A．whether did I look like my mother
B．whether I look like my mother
C．whether do I look like my mother
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