定语从句which, that它们所代替的先行词是事物的名词或代词,在从句中可作主语、宾语等,作宾语时可以省略他们作宾语

很多的!一把说来,关系代词在定语从句作宾语时,在口语中才省,书面语中不省为佳.略举几例：
① This is the pen that / which I like best.
= This is the pen I like best.
② That is the classroom that / which we used to study in.
=That is the classroom we used to study in.
③ The university that / which I studied English in is SISU.
=The university I studied English in is SISU.