21. Apples are usually sold by _______ weight and eggs are sometimes sold by _____ dozen.

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

22. Although punctual himself, the professor was quite used ________ late for his lecture.

A. to have students¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡B. for students’ being

C. for students to be¡¡¡¡¡¡¡¡¡¡¡¡¡¡ D. to students’ being

23. When people move to another country, they often try ____ the customs of their native land.

A. to keep on¡¡¡¡¡¡ B. to keep off     ¡¡¡¡C. to keep up¡¡¡¡¡¡¡¡¡¡ D. keeping up