问题补充：
Each of the students, hoping to pass the exam, the book.
A. buying
B. having bought
C. should buy
D. to buy
答案：
A. buying
B. having bought
(答案→)C. should buy
D. to buy
解析：each of the students是句子主语，横线处为句子谓语，hoping to pass the exam是修饰句子主语的非限制性定语。
