She had three sons,all---became doctors.

She had three sons,all___became doctors.
a.of which
b.which
c.of whom
d.who
请问这题是考什么知识点的?要选择哪一项?请说明原因,谢谢!
那主语是she谓语是had宾语是？all___became doctors在句中是充当宾语吗？