在定语从句里,什么情况引导词可以为“ / "?

1.关系代词（即你所说的引导词）在从句中作介词宾语时,例：The player (whom/that/who) I beat in the table tennis game was a seed player.
2.当关系代词在从句中作介词的宾语,例Here is the man (that/who/whom) you have been looking for.
3.当关系代词在从句中作表语时,例Shanghai is no longer the city(which/that) it used to be.
以上情况引导词可省略