这个定语从句中的先行词是a doctor吗？怎样判定先行词？
问题描述：
His sister has become a doctor,____ is what he wants to be. A who B whose C that D which
答
同学，你好。前面这句话做先行词，就是前面这句话表述的事实，他想要达到的程度
His sister has become a doctor,____ is what he wants to be. A who B whose C that D which
同学，你好。前面这句话做先行词，就是前面这句话表述的事实，他想要达到的程度