单项选择题

（）the reactions of people with this drug, scientists have learned more about the brain.

A. By studying
B. To study
C. They study
D. They're studying

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单项选择题
It was my father' s wish in his will that I（）become an engineer.

A. should
B. would
C. might
D. could
单项选择题
We have produced twice as much steel this year（）4 years ago.

A. than we did
B. as we did
C. than we have produced
D. as we have produced
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