it's price 和 the price of which的区别

问题描述:

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?

要是选c,那么这个句子不再是定语从句(没有了引导词),只能看做并列句,可是并列句缺少连词,这里可以这样说
Recently I bought an ancient chinese vase and its price was very reasonable.