单项选择题

By the time Neil Armstrong and Edwin Aldrin walked on the moon in 1969, there ________ many space explorations.

A．will have been
B．had been
C．have been
D．must be
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单项选择题
I regret __________ hard at school, or I would have passed the exam.
A．to have not worked
B．having not worked
C．not to have worked
D．not having worked
单项选择题
No sooner had we reached the top of the hill __________ we all sat down to rest.
A．when
B．then
C．than
D．until
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