为什么一定要说hand it in?

为什么一定要说hand it in? Is it kept in hand now?为什么不是keep 是hand in it 还是hand it in It's in your hand homework用hand it in 还是hand them in 一些英语题目（一定要说为什么） 为什么一定要说“条条道路通罗马”? there is (某人)in the room ,who can it be 根据汉语提示填空.要说为什么? Please check your homework before handing it in!中hand为什么用ing with do it in the hand put it in your other hand you hand it in 的中文