He showed me a book, ______ I could tell that it was pretty old.
A. which cover
B. of which cover
C. the cover which
D. from the cover of which
网友应该知道非限制性定语从句的结构，也应该知道以下两个事项：
1. the cover of which = the cover of a book
2. tell A from B：区分A和B；tell +that 从句 + from... 从...中看出，说出，知道某个事实。
