单项选择题

Many people proposed that a national committee be formed to discuss ______ to existing mass transit systems.

A．substitutes
B．measures
C．duplicates
D．alternatives
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单项选择题
There are a few small things that I don't like about my job, but ______ it's very enjoyable.
A．all at once
B．once and for all
C．so much as
D．by and large
单项选择题
These pollutants can be ______ hundreds and eve thousands of kilometers by large air masses.
A．contained
B．conveyed
C．contaminated
D．conserved
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