knock on和knock at用差别么?

knock on和knock at用差别么? knock at 和knock on怎么用? knock on;knock at;knock over;knock knock on knock into knock at knock up knock at和knock on有区别吗? Knock on 和Knock at 有什么区别啊? knock at 和knock on 有什么区别? 老师您好!knock on和knock at到底有区别么?有的话,那区别是什么? knock on与knock at的区别 knock into sb和knock at knock at a door和knock on a door的区别是什么 knock at the door 和 knock on the door有什么区别吗