单项选择题

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

A. to be shown
B. to have shown
C. to have been shown D. being shown
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单项选择题
Whatever the causes, English is currently more widely spoken and written than any other language
A. ever was
B. has ever been
C. had ever been
D. would ever be
单项选择题
______ your opinions are worth considering, the committee finds it unwise to place too much importance on them.
A. As
B. Since

C. Provided

D. While
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