I. 单项选择。（每小题1分，共15分）
从每小题所给的A、B、C、D四个选项中选出可以填入空白处的最佳选项。
( ) 21. Computers can be used _________ sending e-mails.
A. as B. for C. to D. by
( ) 22. The number of teachers in our school _____ 60 and a number of them ______ women teachers.
A. is; are B. are; is C. am; are D. be; are
( ) 23. — Who ______ the battery-operated slippers invented
______?
— Julie Thompson, I think.
A. was; to B. was; for
C. were; by D. were; with
( ) 24. — When will you leave?
— I won’t go ________ he comes back.
A. until B. while C. when D. why
