knock at a door和knock on a door的区别是什么

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 there was a knock on the door. There came a knock on the door knock the door和knock on the door都对吧?有区别 knock at the door they were i______ by a knock at the door No sooner_____asleep than she heard a knock at the door 可以说there is a knock at the door吗 A knock on the door!Who --it be? 1.S_____ there was a knock on the door. knock on与knock at的区别