Someone is knocking at the door, but whom can_____be?

Someone is knocking at the door, but whom can_____be?
A、one
B、he
C、it
D、she
【正确答案】：C
【题目解析】：本题考查代词的用法。句意：有人在敲门，但是会是谁呢？在不确定对方是谁的语境中，要用it代指。故本题选C。