请老师讲一下这道题

请老师讲一下这道题


This problem will be discussed next month, by _______ the school leaders will have to make up their minds about what to do with the
problem.
A when         B which           C that          D when
为什么选when 不选which啊

which代表的是名词，也就是problem,但根据句意应该是在这时问题将被解决，这是正确的理解。

A,D都是when吗？which引导非限制性定语从句前面不能加by或其他介词。