some of whom are businessmen.这样可以理解吧.
在定语从句的考点：代词/名词/数词+of +which(先行词指物)/ whom(先行词指人),这个结构里,常会把前面的代词/名词/数词放在关系词的后面,就如你这题.其他的如：
I invited ten friends to have dinner , three of whom let me down.也能讲成：of whom three let me down.
选择D.
一个很难解释的高考英语单选题---语法高手进---不会不要作答,谢谢