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

我认为从句中不缺主语. 宾语,所以B .C . 答案排除.

对呀     从句不缺成分，不能用that，which和who     只能用从句相对应的那个  比如when  
where   why....