OreoShake , while I agree that the commas are overdone in D, it's also true that there are very few absolute rules in English grammar! Like it or not, sometimes writers--even GMAT writers!--will insert a comma that doesn't seem necessary. Remember that commas can serve to indicate a pause in speaking and are sometimes used to break up a sentence for greater clarity. Honestly, I was tempted to put a comma before "and are sometimes" in that last sentence, even though it's not the standard!
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