Scientists say it may be five or six years ______ this medicine is tested on human beings.

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问题 Scientists say it may be five or six years ______ this medicine is tested on human beings.

选项 A、since
B、before
C、after
D、when

答案B

解析 since:自从,因为,由于;before:在……之前;after:在……之后;when:当……时。根据句意可知,选B。
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