21. Children who are not active or diet is rich in fat will gain weight quickly.
A. what B. whose C. which D. that
22. Sam _____ some knowledge of the computer just by watching others working on it.
A. brought up B. looked up C. picked up D. set up
23. We were about to go out for a walk ______ it began to rain.
A. while B. before C. as D. when