【正确答案】 B

【答案解析】couldn’t have done sth．可用来表示对过去某一动作的否定推测，意为“不可能做过某事”，例如：He couldn’t have committed that crime because I was with him when themurder happened．他不可能犯罪，因为凶案发生时我和他在一起。ought to have done意为“本应该做(而实际上未做的)某事”，含有责备之意，例如：You ought to have helped himyesterday．昨天你本该去帮助他。shouldn’t have done sth．意为“本来不应该做某事(反而做了)”，含有责备之意，例如：You shouldn’t have told him about his illness．你本不该把他的病情告诉他。根据句子结构和下文内容我们可以看出该题应该选B。