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

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

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

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