21. A lot of people worry that         air pollution may lead to changes in         weather around the world.

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

22. — Why do you look so angry?

— He insists on my presence here, but I can see it makes no difference ______.

A. I am here                  B. I was here                C. me to be here            D. my being here

23. He was so angry at all ______ she was doing ______ he walked out.

A. what; as              B. that; that           C. that; which        D. what; that