She had two sons,______ became teachers.A,both of them B both of whom 这两个答案到底有什么区别?

题目

She had two sons,______ became teachers.A,both of them B both of whom 这两个答案到底有什么区别?
为什么选B 不选A 呢

答案

B both of whom
因为一个单句里只能有一个主干
She had two sons句子已经完整
所以如果选A,句子就有两个主干,错误
选B,定语从句修饰sons