There is someone outside the door . Go and see who------is.

选 C,原因：it用于指人主要用确定未知人的身份：
“Who is it?” “It’s me.” “是谁啊?”“是我.”
“Listen.Someone is crying.” “Oh,it must be Mary.” “听,有人在哭.”“噢,一定是玛丽.”
Someone must have been here.But we have no idea who it was.一定有人来过,但我们不知道是谁.
为什么不能选A 原因很简单this is这个是（特指） ,正常的用法：通常是用于回答别人或者告诉别人一件事的时候 This是放在前面.
This is my pencil.
This pencil is mine.