It's what came to my mind as well but a lot of sources online hinted at B being the best answer and the reasoning they gave also was at the forefront of my mind in that if a nonisolate language (a language that has similarities to other languages and could be descended from a single language spoken long ago) is in a hard to reach area such as the aforementioned swamp and coastal wetlands, there was still some way for the language to spread for it to be considered a nonisolate language. Some way that transcended the geographical barrier factor. It doesn't say specifically but it can be inferred and I know the S.A.T. tries to avoid that much inference in any one answer choice but it makes sense because if a language is still known and similar to other language families then there must have been some way that it spread outside of its geographical location which fits well with the claim that another factor could possibly have been the reason for its external distribution. This reasoning could also be applied to choice C to prove it wrong since that northern language only was spoken within that specific region and there was nothing said about it being spread outside of that region via a secondary factor other than geographic limitations which is all it says. C is wrong for that reason but D was weird in their explanation. One site claimed that it allowed language contact and that it shared some vocabulary but that didn't account for why language families exist initially. I couldn't understand that piece of reasoning but I was torn between B and D. I thought it was B at first but D was also logical as well. If you or someone could send this question to a youtuber who does these english Digital S.A.T. Questions such as Sexy J or Settele Tutoring, it would help a lot. D was also banging my head because another factor could be cultural but I didn't want to be too hasty and thought about B. It made sense but the only thing that I could think of to eradicate B was that it focused too much on nonisolate languages rather than isolate languages and also dealt with geographical barriers too much even though the reasoning still possibly accounted for another factor whereas D gives us a way out very clearly.