We must do what we can to the river .
A. protect; against polluting
B. stop; polluting
C. keep; being polluted
D. prevent; from being polluted
网友回答
A. protect; against polluting
B. stop; polluting
C. keep; being polluted
(答案→)D. prevent; from being polluted
解析：考查固定词组。固定词组：prevent sb from doing sth阻止某人做某事；句意：我必须坐我们能做地阻止河流被污染。根据句意说明存在着被动的关系，故使用动名词的被动语态。故D正确。