定语从句中什么时候whose=of which

定语从句中什么时候whose=of which
帮看看下面两个问题,
语法参考书上写 of which=whose,请问什么条件下等于?有道题是这样的；
We are talking about the subject _____ importance I haven't realized.
A.which B.that C.of which D.whose
帮看看这句；
The family _that_ I am staying with lives in town.
答案填that
The family 属于人还是什么?
这里只能填that 如果当人来算的,还可以填whom 、who啊,是吧.
什么单数，整体....就要用that
如果这样看 I am staying with whom 不行吗
凡是句子成分不缺了，就只能用that