Scientists say it may be five or ten years ______ it is possible to test this me

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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、when
C、after
D、before

答案 D

解析 before作从属连词，引导状语从句，意为“到……之前”，如：It was some time before I realized the truth．过了很长一段时间我才了解到真相。

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