1、当 "which" 用作关系代词时。
通常会被介词如"in","on","at","by","with"等所预置。
使用的介词取决于上下文和 "which" 之前的动词。
2、介词+which，怎么确定介词的举例：
"on"+which是用于指正在工作的书：
"The book on which I am working is very interesting." (我正在工作的那本书很有趣)
"in"+which用于指说话者泛塑出生的城市：
"The city in which I was born is very beautiful." (我出生的那个城捧汽市非常耻酱愁美丽)
"with"+which用于指说话者去参加聚会的人：
"The person with whom I am going to the party is my best friend." (我和谁去参加聚会是我最好的朋友)