从A、B、C、D四个选项中，选出可以填入空白处的最佳选项。
21. ---Would you mind giving your advice on how to improve our learning efficiency ?
--- If you make _______ most of forty-five minutes’ classes day after day, there will be _____ switch in grades.
A. 不填; 不填 B. the; a C. 不填; a D. the; 不填
22. _______ have an internal clock that controls the beating of our heart and the _______ of our breathing.
A. Human being, steps B. Human beings, steps C. Humans, pace D. Man, pace
23. _______ on your study, and you will make greater progress.
A. Concentrating B. If you concentrate
C. To concentrate
