关于of whom与who

I have many friends,_____some are businessmen
A.who B.of whom
选什么?为什么？，of whom 可不可以作主语，老搞不清它的用法

who与whom区别 who与whom的用法 如何区分who与whom who whom whom与who在定语从句中用法的区别 who and whom who,whom的用法 who whom whose 什么时候用who whom who whom that的区别