I have finished a large part of the book,the rest of which___ more difficul

题目

I have finished a large part of the book,the rest of which___ more difficul
最好给我一些解释啊

答案

is
我完成了书的很大一部分,剩下的（书的一小部分）更难
一本书的一小部分,肯定是单数,所以用is