It was three years he came back.A. thatB. sinceC. beforeD. when

问题补充：

It was three years he came back.

A. that

B. since

C. before

D. when

答案：

A. that

B. since

(答案→)C. before

D. when

解析：考查连词的用法区别。若选 “since”,则主句应为现在完成时；若选“that”则 “three years”要带有介词变为时间状语才能用于强调句对其进行强调；D项如果It was 之后为时间点，且it为时间，则可以选；it was+时间段+before 从句，且前后时态要保持一致，表示过多久才、、、；故C正确。