it's price 和 the price of which的区别

it's price 和 the price of which的区别
Recently I bought an ancient Chinese vase,_______ was very reasonable.
A.which price
B.the price of which
C.its price
D.the price of whose
我知道答案是B,考的是定语从句.但是答案C怎么不行?
类似的还有some of them和some of which; 以及both of them和both of which的区别
比如：We have three foreign teachers,__________are from Canada.（期中的两个）
答案应该是two of whom,而不是 two of them.为什么?
希望英语达人们给出详细的解答.