Reports showed that it was not the first time this state-owned firm ________ serious accidents．
A.has happenedB.had happenedC.has experiencedD.had experienced在线课程D
解析：根据固定句型It is/was the first time that…has/had done sth．判断，排除A、C；happen为不及物动词，后不接宾语；experience（经历）为及物动词，后接宾语。故选D．