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

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

试题答案

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。