ago前面用介词吗

前面不用介词，例如：

1、How long ago did you buy it?这东西你是多久以前买的

2、The house was pulled down long ago.那栋房子很久以前就被拆掉了。

3、Just a few years ago the picture was very different.几年前的情况就大不相同。

4、I used to smoke but I gave up two years ago.我过去抽烟，但两年前就戒了。