It __ five or ten years before the new medicine is tested on

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It __ five or ten years before the new medicine is tested on human beings.
A.will take
B.took
C.takes
D.has taken
为什么不选A或C?
说错了，那为什么不选C或D啊？

这句话是由before引导的时间状语从句.当主句用将来时,从句总是用现在时；如果before引导的从句谓语用的是过去时,则主句动词多用过去完成时,这样以便体现动作发生的先后.由before引导的从句只有这两种情况.在这句话里,从句是现在是,主句用将来时.所以选A.

参考资料：爱问知识人

再问：既然你说了，before引导的主句动作的发生在从句之前。那为什么“当主句用将来时，从句总是用现在时”，这样不是有说明了“before主句动作的发生在从句之后了”。你这两句话不是矛盾了啊？
再答：主句用将来时，从句用现在时，这个用法并不是用来体现动作发生的先后。一般过去时跟现在时的这两种用法之间完全没有联系的。那两句话在讨论两个完全不同的用法。 It __ five or ten years 看看你这道题难道你能说five or ten years是个动作么？

It_________five or ten years before the new medicine is test 英语翻译Scientists say it may be five or ten years ______ it is scientists say it may be five or six years before it is poss It's been five or ten years from happenning for about 50 yea The Kings had lived in New York for ten years before they ca .I had been working on it for five years before we got to th It is five years since we began to enjoy a ten-day spring ho This is just ten years after it reached five billion这句话怎么翻译? 英语短文改错,My father,who worked on a farm ten years before ,is n It is ten years since he ____(join) the army Your bike looks nice.Is it new?No I ()it for five years.A ha It is ten years 是before还是since还是until he come back