—Oh,there's someone knocking at the door.—______ must be that naughty boy.其中,为什么用It?不用He?

—Oh,there's someone knocking at the door.—______ must be that naughty boy.其中,为什么用It?不用He?
—Oh,there's someone knocking at the door.—______ must be that naughty boy.
其中,为什么用It?不用He?

—Oh,there's someone knocking at the door.—______ must be that naughty boy.其中,为什么用It?不用He?
因为不知道是谁.所以不能用he,or she.it 可以表示人物.
英汉词典的解释：
it：〔指心中记着或成为问题的人物、事情和行为〕.Go and see who it is.去看看是谁?It's me.〔口语〕是我 (＝It is I.).Oh,it's you!哦,是你呀!It's the girls having their rehearsal.是女同学在排戏.

It,

It 是固定用法 你不知道那个人是男是女

表示猜测的，不明对象的猜测~