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
