By the time he was twelve he a chemistry lab of his own.A. builtB. has builtC. had builtD

问题补充：

By the time he was twelve he a chemistry lab of his own.

A. built

B. has built

C. had built

D. was building

答案：

A. built

B. has built

(答案→)C. had built

D. was building

解析：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。