is estimated that a round-trip to Mars would take more than a year.A,That B,what C,There D

发布时间：2020-08-17 02:22:40

    is estimated that a round-trip to Mars would take more than a year.
A,That       B,what       C,There      D,It

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解析：通过分析句子结构，可知句子的真正主语是that引导的主语从句，这里需要形式主语，故用it。
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