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：选B。“It + be + 时间名词+before / since”从句句型中，连词before 与since 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，其句间是“自从……以来已经多久了”，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)。