She wrote a month ago we’ve heard nothing.A. since when B. before whichC. since thatD. a

问题补充：

She wrote a month ago, we’ve heard nothing.

A. since when

B. before which

C. since that

D. after which

答案：

(答案→)A. since when

B. before which

C. since that

D. after which

解析：

此题答案选A，其中when 相当于 a month ago。若改为 and since then 也不可以。