题目内容：

句子成分辨别
Who is it?主语是什么?我们老师说主语是it
而Which is the most important invention?的主语却是which
为什么同是特殊疑问句而一个做主语另外一个是表语?
原题：There's someone knocking at the door.Do you know (who it is)?
Which do you think (is the most important invention)?
括号中是答案