Oh there is someone knocking at the door must be your neighbour Mrs Black. A.

问题补充：

--- Oh, there is someone knocking at the door.

--- must be your neighbour, Mrs Black.

A. There

B. She

C. He

D. It

答案：

A. There

B. She

C. He

(答案→)D. It

解析：句意：上句，有人在敲门，下句，你定是你的邻居Mrs Black.在打电话或敲门是不能分辨对方是谁，我们用it来指代，故选D。