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

試題分析：通過分析句子結(jié)構(gòu)，可知句子的真正主語是that引導的主語從句，這里需要形式主語，故用it。
點評：動詞不定式，動名詞或主語從句作主語時，通常把它們放在謂語動詞前，而用it作形式主語，放到句首。
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