敲门用at还是on

　　I knocked but there was no one around.

　　我敲了门，但是没有人应门。

　　The two rooms had been knocked into one.

　　那两间屋子打通了，成为一间屋。

　　He knocked his opponent down three times in the first round.

　　他第一局就将对手击倒三次。

　　They had to knock the door down to get in.

　　他们不得不破门而入。

　　I knocked on the door but there was no response.

　　我敲了门，可是没有回应。