2. 外语
  3. It takes at least five to ten years ________it is possible to test this medicine on human patients.

It takes at least five to ten years ________it is possible to test this medicine on human patients.

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问题 It takes at least five to ten years ________it is possible to test this medicine on human patients.
选项 A、before
B、since
C、after
D、when
答案A
解析 本题考查时间状语从句。本句是before引导的时间状语从句的固定句型：it+谓语+some time+before sb．do(did)sth．，只有A项before放入句中符合。
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