knock at,knock on

knock on;knock at;knock over;knock knock on knock into knock at knock up knock on与knock at的区别 knock at 和knock on怎么用? knock at和knock on有区别吗? Knock on 和Knock at 有什么区别啊? knock on和knock at用差别么? knock at 和knock on 有什么区别? knock knock into sb和knock at knock at the door knock at的意思