題目：
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
答案給的是C，先行詞是book，為什麼會是用whose呢？還是這裡不是先行詞的觀念呢？

這裡是關代所有格的用法 書"的"封面

謝謝樓上解答！