She raised her finger to her lips as _____ for silence.
A.an idea
B.a mark
C.a sign
D.a word
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C解析应选C，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