(2013·湖南高考)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
网友回答
A. wouldn't
B. shouldn't
(答案→)C. couldn't
D. mustn't
解析：选C 考查情态动词。句意：尽管他想睡，但仍睡不着，(因为)当时他拼命想着一个主意，一直到想出来为止他才睡去。此处couldn't表示“不可能”，符合句意。