单选题 3.  Sally couldn't have written to Frank, or he ______ received her letter.
【正确答案】 C
【答案解析】 句子意思是：萨莉不可能给弗兰克写过信，否则他应该已收到她的信。鉴于此句前半句是对过去发生的事的否定推断，故后半句应用虚拟语气would have received。