单项选择题

Somebody in the next room must be ill,（）a doctor has just come out.

A．because
B．for
C．in that
D．as

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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
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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