单项选择题

The largest system serving e-mail messengers is the Internet, a ______ of millions of computers linked worldwide.

A．unity
B．combination
C．network
D．connection
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单项选择题
There is not a conductor ______ resists the flow of the electrical charges.
A．which
B．but
C．that
D．it
单项选择题
Scientists think ______ helps some trees to conserve water in winter.
A．when losing leaves
B．leaves are lost
C．that losing leaves
D．the leaves losing
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