摘要：760] By the time he was twelve he a chemistry lab of his own. [译文] 到他12岁的时候.他就建起了他自己的实验室. A. built B. has built C. had built D. was building [答案及简析] C. 有by the time 引导的短语或从句.主句应该是完成时态.至于是什么完成时态.就看by短语或从句中的时态了.

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