A hinge joint is what permits the forward and backward movement of a door. 这是表语从句吗？

Question

为什么有人说是主语从句？
http://zhidao.baidu.com/question/171556149.html

Answer 1

当然表语从句。
是主语从句的话，A hinge joint 是何成分呢？追问

i think it is () you are doing too much.
Abecause of B because Cbecause that Dbecause for
为啥选B。。。。

追答

Abecause of 用于后面接短语（非句子）
Cbecause that： because已经是引导词。
Dbecause for：无此用法