It was quite a long time I made it out what had happened.
A. after
B. before
C. when
D. since
网友回答
A. after
(答案→)B. before
C. when
D. since
解析：在“It+be+时间名词+before/since”从句句型中，连词before与since极易混淆。用before引导时，句意是“过了多久才……”，主句中的be动词为过去时或将来时态；用since引导时，其句意是“自从……以来已经很久了”，主句中的be动词多为现在时（或现在完成时）或一般过去时（此时since从句要用过去完成时）。