单项选择题

Oxygen is often stored as a liquid, although it is used () as a gas.

A、primarily
B、seldom
C、rarely
D、sometimes
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Under no () should we be reckless of consequence and make hasty decisions.
A、instance
B、circumstance
C、situation
D、environment
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I have heard () he will come tomorrow.
A、when
B、how
C、what
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