单项选择题

Scientists everywhere are trying to () a cure for AIDS.

A. expect
B. invent
C. discover
D. make

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单项选择题
The orchestra () for about ten minutes when a man in the audience suddenly began shouting.

A. was playing
B. had been played
C. had been playing
D. was played
单项选择题
No sooner had we reached the top of hill () we all sat down to rest.

A. when
B. then
C. than
D. until
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