Once you have cured the water pollution problem,you will have to take steps to avoid it___again,
问题描述：
Once you have cured the water pollution problem,you will have to take steps to avoid it___again,
多选题：A.to happen; B.happening; C.happens; D.happened,
选A和B不对，如果单选，应选哪个？
答
A.to happen; B.happening多选么?
avoid to do/sth/ doing