21. The invention of_______ paper was a great contribution to _______ human civilization.
A. a; a B. the; the C. /; / D. the; /
选C。考查冠词的用法。paper为物质名词，前面不加冠词；human civilization“人类文明”，习惯上前面也不用冠词。
22. All the students should be _______ of their school and not do things that endanger the school’s reputation.
A. proud B. afraid C. popular D. frightened
选A。句意：任何学生都应该以学校为荣，不做影响学校声誉的事。be proud of“以……为自豪”，符合句意。be afraid of“害怕”; be popular with“受……欢迎”；be frightened of“害怕”。
