一道高考英语单选I was told that there were adout 50 foreign students

一道高考英语单选I was told that there were adout 50 foreign students2006辽宁高考英语选择题:I was told that there were adout 50 foreign students----Chinese in the school,most---were from Germany.A study,of whom B study,of them C studying,of them Dstudying,of whom为什么第二空用of whom不用of who?因为介词of吗?应该也可以用of which吧,我知道不用of them