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
