2. 外语
  3. Some Martian rock structures look strikingly like structures on Earth that are known_____ by microbes.

Some Martian rock structures look strikingly like structures on Earth that are known_____ by microbes.

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问题 Some Martian rock structures look strikingly like structures on Earth that are known_____ by microbes.
选项 A、having been created
B、being created
C、to have been created
D、to be created
答案C
解析 空格所在的定语从句为“sth． be known to do”结构，因此排除A、B项。其次，that指代地球上的岩石结构，与created之间存在被动关系，故要用被动式，C、D都符合。最后，当不定式动作发生在谓语动词的动作之前时，不定式需要用完成式，本题中，在人们已知前，岩石结构已经由微生物创造成，即created动作先于are known发生，所以选择C项to have been created。
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