By the time Einstein was 14 years old, he himself advanced mathematics.A. has taughtB. ta

发布时间：2020-08-17 14:01:40

By the time Einstein was 14 years old, he      himself advanced mathematics.
A. has taught
B. taught
C. had taught
D. was teaching

网友回答

A. has taught
B. taught
(答案→)C. had taught
D. was teaching
解析：by the time 引导时间状语从句，主句中的动作发生在从句中的动作之前。故用过去完成时。
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