there are 30 volunteers,many of whom come from a university.

there are 30 volunteers,many of whom come from a university.
为什么不用which,that,who

这是定语从句,先行词是volunteers,志愿者是人,所以应该用可以指代人的引导词,所以which不对.而定语从句用that的条件是从句中不缺成分.很明显,从句中come from a university没有主语,它缺主语成分,所以不对.
many of ,许多的.其中of是介词,后面接人的时候接宾格,本应该说成many of them,who用于主格,whom用于宾格,所以是whom

这句话的意思：有30名志愿者,他们大多来自于一所大学.