从A、B、C、D四个选项中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。
21. The Qing Ming Festival, ________ festival to hold memorial ceremony for ________ dead, is coming.
A. /; the B. a; a C. the; a D. a; the
22. --Here is paul! We ________ anxiously for him since last night.
--Maybe something happened to him.
A. waited B. were waiting C. had waited D. have been waiting
23. CO2 is a major cause of global warming, but the pollution natural gas causes is only 1/800 of ________ caused by burning coal.
A. that B. it C. one D. al
