21. Many people have come to realize that they should go on ______ balanced diet and make ______ room in their day for exercise.
A. a; / B. the; a C. the; the D. /;a
22. The physics problem remained confusing to the boy although ______ over and over.
A. explaining B. being explained C. explained D. having explained
23. You ______ lose the chance of taking the examination of this kind in the next three years if you dare to cheat!
A. can B. shall C. should D. may
