单项选择题

Scientists say it may be five or ten years（）it is possible to test this medicine on human patients.

A. since
C. after
B. before
D. when

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How long does the training period last Two ______.
单项选择题
— I think it is going to be a big problem. — Yes, it could be. I wonder（）we can do about it.

A. if
C. what
B. how
D. that
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