21. In our class，the student leaders are on ________ duty every ________ few days.
A. the; a B. /；a C. /；/ D. the；/
【解析】选C。考查冠词及固定搭配。on duty“值日/值班”，是固定短语。every few days表示“每隔几天”，是固定表达。
22. This year his family have earned ________ as they did last year.
A. twice as big an income B. as twice big an income
C. a big income as twice D. twice an income as big
【解析】选A。考查比较级。句意为：今年他家的收入是去年收入的两倍。倍数要放在as. . . as. . . 的前面。
