求一到高中英语难题

我们老师发给我们的英语单选题~~

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
24.—The experiment is of particular importance.
—I see. We will carry on with it ________ we can get enough money.
A. unless          B. though  &nb