Some Martian rock structures look strikingly like structures on Earth that are known ______ by microbes.
A. having been created
C. to have been created
C是正确答案，可是A和C都可以表示非谓语的动作先于谓语动词的动作，为什么不能选A?
sb/sth is known to do/to be doing/to have done. 这个句型只能后接不定式，不可以用现在分词。要了解动词的用法，以及句型结构，不能想当然。sb/sth is known/believed/reported/said/though/etc, 后接不定式结构。
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