例：A computer can only do ______ you have instructed it to do.
A. how B. after C. what D. when
答案是C．
21．—I just received a letter from one of my old school friends.
—______
A. How come? B. You bet. C. Congratulations! D. That’s nice.
22．Chongqing is ______ most beautiful mountain city and I’m sure to come for ______ second time.
A. the; a B. a; a C. the; the D. a; the
