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
The student I am teaching this year is much more intelligent than（）I taught last year.

A. that
B. one
C. the one
D. those
单项选择题
The reason why John left his home was（）he felt he'd brought shame to his parents.

A. because
B. that
C. as
D. for
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