Hey Osirus,
Sorry to miss this earlier in the week - I was traveling to meet up with some of our instructors back east (incidentally, a fantastic group of people!).
One thing you might consider doing in this case is to negate each of your remaining answer choice to see if it has the opposite effect on the statement. The GMAT is great at hiding behind 'negation' to obscure correct answer choices.
Say a question asked:
Experiments suggest that a fifth universal force of mutual repulsion between particles of matter exists in the universe.
Which of the following most strengthens the argument above?
A) No previously established scientific results are incompatible with the existence of a fifth universal force.
B) No scientists have suggested that the alleged fifth force is an aspect of gravity rather than being fundamental in itself.
Notice that both answer choices involve negation - "no" - behind which the GMAT authors can hide a bit. If you negate them, you'd find that:
A) SOME previously established scientific results are incompatible with the existence of a fifth universal force.
B) SOME scientists have suggested that the alleged fifth force is an aspect of gravity...
Well, the first one, if negated, definitely weakens the theory - there is scientific evidence that it does not hold. Because we know that, either way, the statement will impact the conclusion, we know that it's within the scope of it. The second is a little softer, and shown to be out of scope. "Some" just means "not none", and "suggested" is a pretty soft statement...it doesn't show that there is any proof - if one scientist had theorized that this fifth force was actually just a subset of gravity, that doesn't mean that he's correct. Therefore, the answer here is A.
See if that strategy helps when you're down to two, particularly as you attempt to determine "in scope" vs. "out of scope". Also, for narrowing down to two and then deciding from there, I've found the following strategy to be really helpful:
https://blog.veritasprep.com/2009/11/gma ... ek_20.html