It's a leap from "may often be presumed" to "clearly." Think about the following examples:
My parents don't like dogs because they bark.Here, "they" clearly does
not refer to the subject of the first clause--it's the dogs that bark.
My parents support charities because they are good people.Here, "they" clearly refers to the original subject--the charities are not "good people."
My parents had lunch with my grandparents before they left town.Here, it's really hard to tell the author's intent. From the sentences above, we see that such a pronoun could go either way. A good writer avoids this kind of ambiguity, so the GMAT won't have it in a correct sentence. The other two sentences could easily be correct answer choices, because there's no real ambiguity.