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

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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
单项选择题
I don't think I'll need any money but I'll bring some（）

A．at last
B．in case
C．once again
D．in time
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