单项选择题

Some people viewed the findings with caution, noting that a cause-and-effect relationship between passive smoking and cancer remained ______.

A. to be shown


B. to have been shown
C. to have shown


D. being shown
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单项选择题
I’m sure he is up to the job ______ he would give his mind to it.
A. if only
B. in case
C. until

D. unless
单项选择题
He ______ in a small workshop, but now he is president of a big business company.
A. used to working


B. was used to work
C. used to work


D. was used to working
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