21. Because so many visitors are visiting our school today, some teachers have to _______ guides.
A. serve as B. regard as C. treat as D. use as
22. He came back late, _____ which time all the guesses had already left.
A. by B. at C. during D. after
23. The name of our country is PRC, which _____ the People’s Republic of China.
A. stands by B. stands for C. stands up D. stand out
