He ran downstairs and rushed to ———— his mother was sittingA,the place which B,which C,where D,the place 为什么答案选C不能选A?

这个是where引导的状语从句，不是定语从句。如果要选择A，那么从句应该是his mother was sitting on/in ，先行词是the place ，which做从句中介词的宾语。

选C
his mother sitting in the place 现在没有了介词 就要用where关系副词

A是定语从句，但关系词不能用which 要改成the place where

C,where
他跑下楼,冲到他妈妈坐的(地方)
如果用A的话,要改一些,而且,这时候, sit 要用及物的了_不然,怎么会有宾语的?
He ran downstairs and rushed to the place in (/at? 因为没见过这么问的,还真不知道用哪个介词好,但一定要用的) which his mother was sitting

这个解释很简单啊，因为sitting修饰的肯定只能是地名啊，而位置的代名词就是where啊~~其实C答案你可以理解为省略了the place（where)

A项缺介词。

sitting是不及物动词,如果接宾语必须先接上介词.

I was listening to (what he was singing).
这里what he was singing做前半句的宾语从句，what做singing的宾语。
如果指时间可以说
I knew (when he arrived).
如果指地点可以说
I knew (where he lived).
那么本题就是
He ran downstairs and rushed to （where his mother was sitting）
where 是括号部分的地点状语
以上属于宾语从句，以下属于定语的内容
A项应该是
He ran downstairs and rushed to the place（ at which his mother was sitting）