21. He has _______ great interest in history, especially in _______ history of Tang Dynasty.

A. a; /   B. a; the     C. the; the     D. /; a

22. Is this the reason _________ to her teacher for her being late?

A. she explained   B. what she explained  

C. how she explained   D. why she explained

23. There are no buses. That’s why I ________ walk back home.

A. must  B. have to  C. should   D. can