这是要分情况的。
1、例:He is the first person/the only one /the most friendly person that i have talked to
先行词被序数词,不定代词,最高级修饰时,只能用that.
2、例:He is one of the students to whom i have talked
当介词提前,介词后必须用且只能用whom
3、例:Gina ,who lives in China is my friend.
当定语从句为非限制性定语从句时,只能用who。
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