【题目】He ________ sleep, although he tried to, when he got on such a hunt for an idea until he had caught it.
A.wouldn't
B.shouldn't
C.couldn't
D.mustn't

【答案】C
【解析】句意：当他寻找到他一直想找到的想法时，尽管他想睡，却不能睡着。表“能够”，用can/could。would “ 将，将要；愿意”；should“ 应该〔不问人称，表示义务，责任〕；大概是，有可能”；must“ 必须；必然要，一定”。因此选C。