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有区别吗? 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;knock at;knock over;knock knock on与knock at的区别 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? knoch at 和knock on的区别 朋友们帮忙,请问“knock”的用法,主要说明下“knock on/at”和“knock into”的用法及区别，最好有例句！ knock on knock into knock at knock up knock at a door和knock on a door的区别是什么 knock at 和knock on怎么用?RT knock the door和knock on the door都对吧?有区别 knock做vt.和vi.有区别吗.分别怎么用呢 knock into sb和knock at knock off和knock down的区别 knock over和knock into 区别 hit和knock有什么区别?