◎ 题干
— It's reported that the new underground line has been completed.
— Yes, but it hasn't been made clear _____ it's to be opened to traffic.
[     ]
A. that
B. who
C. what
D. when
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◎ 解析
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◎ 知识点
    根据魔方格专家分析,试题“— It's reported that the new underground line has been completed. — Yes, but it hasn't been made clear _____ it's to be opened to traffic. [ ]A. that B. w…”主要考查了你对  【主语从句】  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。