从A，B，C，D四个选项中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑
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
