21. -- How many students are there in your class_______?

-- Forty-five.

A. at all         B. in total        C. in the end      D. in the total

22. ____ the students in our class is 50 and _____ them study hard.

A. The number of, the number of    B. A number of, the number of

C. The number of, a number of      D. A number of, a number of

23. I was doing my homework _____ a boy rushed into the classroom.

A. when        B. while          C. as             D. for