语法：英语连词的用法之三

英语自学网

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。