第二部分英语知识运用（共两节，满分45分）
第一节单项填空（共15小题；每小题1分，满分15分）
21. China, a country with 145 million elderly people, will remain _______ ageing society during this century, according to _______ Chinese population experts.
A. /; the B. an; / C. an; the D. the; /
22. He devoted most of his time to his research, believing his hard work would ___sooner or later.
A. put off B. take off C. come off D. pay off
