单项选择题

A knock at the door again! It was the third time someone ______ me that evening.

A．has interrupted
B．had interrupted
C．to have interrupted
D．would have interrupted

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单项选择题

This is so difficult a problem that the scientist is known to ______ on it for about ten years, but hasn't found an answer yet.

A．have worked
B．have been working
C．be working
D．work on

单项选择题

Suppose there were a flood or an earthquake and the people could not be reached by trains or trucks. ______ packages of food could be dropped from planes, the people might starve.

A．Until
B．Before
C．Unless
D．If only

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