21. I love ____ history because it gives me ____ knowledge of past events.

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

22. Clouds are gathering. I think we’d better _________ the school in case it starts to rain.

¡¡¡¡    A. run out¡¡¡¡¡¡B. bring up¡¡¡¡¡¡C. refer to¡¡¡¡¡¡D. head for

23. The first edition was published in 1955 and _____ a best-seller ever since.

        A. had been    B. will be     C. would be      D has been