21 .The average price of commercial houses in our city has reduced by _____ 30 percent ,so we can buy one at _____ low price .
A. the; the B. the; / C. /; a D. /; the
22. We must keep our room clean. Dirt and disease go _____, you know.
A. from time to time B. hand in hand C. step by step D. one after another
