单项选择题

（） is often the case; we have worked out the production plan.

A. As
B. Which
C. What
D. While

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Cares play an important role in modern life, but they () also cause many problems like air pollution.

A. can
B. should
C. would
D. need
单项选择题
It is essential that people () enough vitamins every day.

A. have
B. had
C. must have
D. will have
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