Which verb form correctly completes the sentence: 'By the time he arrived, the meeting _____ for 30 minutes.'?
答案解析
核心考点：时态语态。解题思路：'by the time+过去时'需用过去完成时。关键依据：持续时间需用延续性动词。选项分析：D项had been on（会议持续）正确；A项非延续动词；B项时态错误；C项进行时表重复动作不合理。易错点：混淆瞬间动词与延续性动词。
正确答案：D