21.Bill was doing a lot of physical exercise to build up his __________.
A.ability B. force C. strength D. mind
22. I hate _____ when people talk with their mouths full.
A. it B. that C. these D.them
23.The teacher always waits until all the children ______ before he starts the lesson.
A. get down B. set down C. put down D. settle down
