We must try our best to the river being polluted any more.
A. separate
B. keep
C. prevent
D. guard
网友回答
A. separate
B. keep
(答案→)C. prevent
D. guard
解析：句子意思：我们必须尽最大努力去阻止这条河再被污染。Separate分开、（使）分离、区分、隔开；keep保持、保留；guard保卫、守卫；prevent预防、阻止。C选项最合适，且 prevent ( from ) doing sth.阻止做某事，是固定用法。