I have always been honest and straightforward,and it doesn’t matter ______ I’m talking to.

问题描述:

I have always been honest and straightforward,and it doesn’t matter ______ I’m talking to.
A.who is it B.who it is C.it is who D.it is whom
为何答案是B
麻烦解析一下句子结构
who是什么从句做什么语
在to有不该用宾格么?

我总是这样诚实直率,我在与谁交谈根本不重要.
这是一个并列句结构.who it is是做逻辑主语,it doesn't matter中的it是形式主语,I 'm talking to是做逻辑主语的定语.