问题
选择题
By the time Einstein was 14 years old, he ________ himself advanced mathematics.
A．has taught
B．taught
C．had taught
D．was teaching
答案
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。
搜题请搜索小程序“爱下问搜题”