the old man has two sons,both of (whom)are lawyers这是主从复合句,both of (whom)are lawyers是个非限制性定语从句比较：The old man has two sons.Both of (them)are lawyers这是两个独立分句,所以括号里面的用...定语从句中，介词在关系代词之前时， 指人只能用whom, 指物只能用which. 记住！！！第二个我说了是两个独立的句子了， 后面不是定语从句，当然不能用关系代词whom了。 更不能用who了