单选题 Not once did the supervisor ______ during the staff dispute, hoping the employees involved would find a solution for themselves.
【正确答案】 A
【答案解析】[解析] 倒装句
这句话中的否定词Not被提到了句子最前面，主语与谓语采用了倒装。助动词do的过去式形式did出现在句子开头时，主语后面的动词应使用动词原形，故选A. intervene。