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

问题补充：

is estimated that a round-trip to Mars would take more than a year.

A,That B,what C,There D,It

答案：

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。