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