单项选择题

Hardly______ the helicopter______ when the waiting crowd ran toward it.

A．had; landed
B．has; landed
C．did; land
D．was; landing
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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'
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
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