B were they 前面是否定意思所以后要用肯定形式.为什么是复数，前面不是单数吗？no one是不定代词指人。它变为反意问句时用代词they这是固定的。有时也可用he。我把你的整个句子改一下。因为你写错了时态。No one was absent from school yesterday, were they? 这句中前面是否定句后一定是肯定句，只有were they符合，所以其它三个都不对。