三、which可以做疑问代词和关系代词。
1. 用作疑问代词时，意思是"哪一个,哪一些"
Which is my seat? 哪个座位是我的?
2. 用做关系代词时，意思是"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。