单项选择题

The time has come______ we can make extensive use of nuclear energy.

A．as
B．while
C．when
D．since'
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单项选择题
Some metals are better conductors of electricity than others,______ means that the former has atoms that contain more free electrons than the latter.
A．that
B．this
C．it
D．which
单项选择题
Is it necessary to______ our body to the sunlight
A．reveal
B．display
C．expose
D．show
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