英语中的whichthe speed at which the car runs depends on the road

代词 pron。
1。(用作疑问代词)哪一个,哪一些
Which is my seat?
哪个座位是我的?
2。(用做关系代词)那一个,那一些
He also had a gun with which to defend himself。
他还有一把自卫用的枪。
He invited us to dinner,which was very kind of him。
他邀请我们吃晚饭,他这么做真是太客气了。
She was very fond of speaking French,which indeed she spoke well。
她很喜欢讲法语,而且确实讲得不错。
He lives in the house which is opposite ours。
他住在我们对面的那栋房子里。
形容词 a。
1。(用作疑问形容词)哪一个,哪一些
Which university did you go to,Oxford or Cambridge?
你上过哪所大学?牛津还是剑桥?
2。(用作关系形容词)这个,这些
The doctor told him to give up smoking,which advice he took。
医生叮嘱他戒烟,他接受了这一忠告。
再问： 他有一把自卫用的枪，不可以这样写吗：he also had a gun to defend himeself为什么？