题目：
A softbound book is a book ______ a light, flexible cardboard.
(A) that cover is made of
(B) which cover is made of
(C) whose cover is made of
答案给的是C，先行词是book，为什么会是用whose呢？还是这里不是先行词的观念呢？

这里是关代所有格的用法 书"的"封面

谢谢楼上解答！