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 本题为固定用法题。本句意为“科学家们称,可能五六年之后这种药物才能用在人身上”。“It may be…before”表示“可能过多久才……”,故选项B)为正确答案。
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