By the time Einstein was 14 years old, he ________ himself advanced mathematics.
A．has taughtB．taught
C．had taughtD．was teaching
C
by the time 引導(dǎo)時(shí)間狀語從句，主句中的動(dòng)作發(fā)生在從句中的動(dòng)作之前。故用過去完成時(shí)。
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