GMAT Sentence Correction (SC) | Re: A 2009 study from the California State Housing Authority

It's all about the meaning of the list of traits at the end. Only D gets this right. A gets it wrong for the same reason that you wouldn't say "The popcorn was very oily and salty, a bit stale." In this case, the list of traits combines two (relatively small and old), so it's appropriate to connect the last trait with "and." "The popcorn was very oily and salty and a bit stale."
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