Einstein’s theory of relativity seemed hard to believe at the time _
问题描述：
Einstein’s theory of relativity seemed hard to believe at the time _
A.when he first introduced B.that he first introduced it C.he first introduced D.which he first introduced it
答
A,C 缺宾语,introduce 是及物动词,得有宾语
D at which 就对了,单个which 不能做时间状语
B 中的that 等于when= at which,在定语从句中作时间状语
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