It was three years he came back.
A. that
B. since
C. before
D. when
网友回答
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。