◎ 题干
_____ many times, John still couldn't work out the problem.
[     ]
A. Trying  
B. Tried  
C. To try  
D. Having tried
◎ 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“_____ many times, John still couldn't work out the problem. [ ]A. TryingB. Tried C. To try D. Having tried…”主要考查了你对  【现在分词的完成式】  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。