I think by five o'clock the children (C.will have return) from their work.我认为到五点钟之前,孩子们都已经下班回家了.时间状语by five,意为“截止到五点时（已经做了）”,它用于完成时态.而宾语从句的动作是还...哦，那像这种呢：i（）there five minutes when all the lights ().A.had been/went. B.have been/go选择A。从句过去时，主句是过去完成时。