Interesting! I'd love confirmation that this one really did come from GMATPrep - I did some searching, but couldn't find any confirmation of the source. (People sometimes post things on our site under the GMATPrep folder when they don't know the source because they know we won't answer it if they don't cite the source...)
If you have seen this yourself in GMATPrep, please let me know!
This question in the link seems to be testing something slightly different (not just a straight "because vs. since" set-up). It's testing a particular idiom:
X was easy, because of A, B, and C.
X was easy, <since OR because> A, B, and C <caused something else to happen>.
Options A, B, and C don't do one of the above correctly, so we're stuck with D or E (and, therefore, stuck with "since").
If this is really GMATPrep, I'd say that they're loosening their restriction on "since" a bit, bringing it up to modern usage / times. Definitely worth knowing - if that's what's happening, then it's probably the case that they prefer "because" to "since" as long as everything else is correct, but we should choose "since" if the "because" options are all incorrect for some reason.
And, if this is a GMATPrep question, then I highly doubt they'll ever make you choose strictly between since and because (they don't here) - the issue is the construction of the rest of the sentence.
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Stacey Koprince
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