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

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问题 Scientists say it may be five or ten years______it is possible to test this medicine on human patients.

选项 A、since
B、before
C、after
D、when

答案B

解析 本题为习惯性用法题。英语中表示“花多长时间才发生某事”时,常用“it is/was…before”这一结构,即“在……以前花了多少时间”。所以应选择B)选项。
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