Actually I find the comma unnecessary. *Actually, after I wrote this, Jim from Stratus prep corrected me below. So check out the rest of my explanation and then see what he had to say.*

So the answer to your question is that probably there is no good reason for the comma to be there.

Having said that, in writing one often has some leeway in the use of commas, and sometimes one may place a comma somewhere for emphasis.

For instance, one could say, "I ran into Sue and John at the beach." Ok, fine, that makes sense.

At the same time, if one wanted to emphasize that there was something special about running into not just Sue but also John, one might put commas around and John. "I ran into Sue, and John, at the beach."

Does it make sense that the writer of the sentence you are wondering about wanted to somehow emphasize that the people being discussed don't just identify themselves as culturally deaf but also use sign language? Not really. Maybe the writer added the commas to make the rest of the answers choices work.

Marty's explanation about emphasis is correct, but I think the phrasing actually does need to be emphasized here. It is a key component that the families use sign language - the use is what develops language abilities, the topic of the sentence. It would be easy to miss that point otherwise.

Jim@StratusPrep wrote:Marty's explanation about emphasis is correct, but I think the phrasing actually does need to be emphasized here. It is a key component that the families use sign language - the use is what develops language abilities, the topic of the sentence. It would be easy to miss that point otherwise.

Wow, Jim, you nailed it. Too funny, I explained it and then didn't even notice just how well what I explained applies. Maybe I just didn't expect to see such a subtle and well executed use of punctuation in a GMAT preparation question.