19. The teacher told the children to keep the classroom__.

A. to clean    B. cleans    C. clean      D. cleaned

20. He doesn’t smoke and hates women __.

A. smokes   B. smoke     C. smoked      D. smoking

21.-- How often do you clean the fish tank?

   --____.

A. this afternoon   B. only once   C. once a month     D. for two days