i have no idea ( )the boy is doing in the next room 为什么用what 不是that

i have no idea ( )the boy is doing in the next room
have no idea
不知道,不了解； 没谱儿
have no idea
=don't know
I don't know ( )the boy is doing in the next room ,那么肯定是what
一个典型的宾语从句,what在句子里不可以省略,所以不能用that