单项选择题

Carbon, __ is a very important part of living things on the earth, is rare on the moon.

A．what
B．that
C．which
D． it
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单项选择题
__ is not clear.
A．That he will come
B．He will come
C．Whether he will come
D．If he will come
单项选择题
Cooperation should __ for mutual benefit in energy development and utilization.
A．be enhanced
B．enhance
C．have enhanced
D．to be enhanced
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