Mr.Green has two sons,one of____ working as a bus driver now.中间填什么,为什么

题目

Mr.Green has two sons,one of____ working as a bus driver now.中间填什么,为什么
答案给的是them，但是还是不大懂

答案

填whom
注意one前面是逗号,说明前面的句子还不完整,而后面是个定语从句.如果one前面是句号,则填them.
此外,working前面你少打了个is.