第一节：单词填空（共15小题；每小题1分，满分15分）
21. “This is ______ most useful reference book, ” Mr. Lee told us in ______ class.
A. the; a B. a; 不填 C. 不填; 不填 D. a; a
22. —Is this the magazine ______ she often writes articles?
—Yes, it is.
A. to which B. of which C. for which D. about which
