She's the only one who's studying Frence?为什么用who,而不用that?不是说定语从句中有only修饰时,关系词用that么?

She's the only one who's studying Frence?为什么用who,而不用that?不是说定语从句中有only修饰时,关系词用that么?
She's the only one who's studying Frence?为什么用who,而不用that?
不是说定语从句中有only修饰时,关系词用that么?

She's the only one who's studying Frence?为什么用who,而不用that?不是说定语从句中有only修饰时,关系词用that么?
先行词为指人的all,one,ones,someone,anyone,everyone,no one等代词时,用who.

中心词是one,代指人，故只能用who