The man,of whom the red car is parked in front of our house,is a worker.

The man,of whom the red car is parked in front of our house,is a worker.
改错,of whom一定要改成whose?
看到语法书上有这样的句子：There are in this class 20 students,whose/ of whom the backgrounds are different.这句话为什么二者皆可呢？