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¡¡¡¡21. All of ______ sudden, some passengers on ________ board let out screams, running upstairs like _____ crazy.
¡¡¡¡A. a; /; / B. the; the; / C. a; the; / D. the; /; the
¡¡¡¡22. More and more factories have been built around the city, which leads to ______.
¡¡¡¡A. the rivers having polluted B. pollute the rivers
¡¡¡¡C. the rivers being polluted D. the rivers to be polluted