GMAT Sentence Correction (SC) | Many of them chiseled from solid rock centuries ago

Don't worry about pronoun ambiguity as long as the meaning is clear. It's only a problem when we *really* can't tell, as in this example:

When Kelly and Leena were crossing the street, a loud motorcycle startled her.

I should add that there is no problem with the use of passive voice in any of the answer choices.

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