Bob ________ hard, otherwise he ________ in his exam．A.can’t have worked; wouldn’t have failedB.can’t work; wouldn’t have failedC.mustn’t have worked; wouldn’t failD.mustn’t work; would fail
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A解析前半句说Bob当时不可能努力学习，can’t have done；后半句是与过去事实相反的虚拟语气表达：不然他就不至于考试不及格了。