knock at 和knock on怎么用?RT

knock on;knock at;knock over;knock knock at 和knock on怎么用?RT knock on knock into knock at knock up Knock on 和Knock at 有什么区别啊? knock at和knock on有区别吗? knock at 和knock on 有什么区别? knock on与knock at的区别 knoch at 和knock on的区别 knock into sb和knock at knock off和knock knock at the door 和 knock on the door有什么区别吗详解,谢 knock at the door和knock on the door到底有什么区别? knock at a door和knock on a door的区别是什么 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 knock knock knock