◎ 题干
There must be someone at the door. I heard a bell         .    

A. ring  
B. to ring
C. ringing  
D. was ringing
◎ 答案
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◎ 解析
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◎ 知识点
    根据魔方格专家分析,试题“There must be someone at the door. I heard a bell . [ ]A. ringB. to ring C. ringingD. was ringing…”主要考查了你对  【实义动词的现在分词】  等知识点的理解和应用能力。关于这些知识点的“档案”,你可以点击相应的链接进行查看和学习。