She raised her finger to her lips as for silence. A. an ideaB. a markC. a signD. a word

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A. an idea
B. a mark
(答案→)C. a sign
D. a word
解析：考查名词。sign与mark的区别是：sign 的意思是“迹象”、“征兆”gesture or movement made with the hand, head, etc, used to give information, a mand, etc(用手或头等做出示意动作以传递信息或命令等)，mark 的意思是 written or printed symbol or figure, line etc made as signor an indication of sth(书写与印刷的符号或图、线等记号)。根据此二词的语义区别以及常识可知答案为C。