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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单项选择题

Under no () should we be reckless of consequence and make hasty decisions.

A、instance
B、circumstance
C、situation
D、environment

单项选择题

I have heard () he will come tomorrow.

A、when
B、how
C、what
D、that