单项选择题

Some people viewed the findings with caution, noting that a cause-and-effect relationship between passive 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
Mr. Johnson preferred （） heavier work to do.

A. to be given
B. to be giving
C. to have given
D. having given
单项选择题
As early as 1647, Ohio made a decision that free, tax-supported schools must be established in every town （） 50 households or more.

A. having
B. to have
C. to have had
D. having had
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