因为the one前有一个定冠词the,而one前面是“零冠词”,因此the one用that引导,one可以用who或that引导.
当先行词是不定代词,all,none等或先行词前有如the bigist,the first等词修饰时,关系代词必须用that.其余情况下,一般可以把关系代词who换成that.