Nice discussion here, and I really like seekmba's point that both "so X that Y" and "so A as to B" are valid idioms. We can't so much pick a right answer because it has a right idiom; we can only eliminate those answers that we know to be wrong.
This question looks to be a paraphrase of a few Official Guide questions so it may have its own explanation, but I'd be shocked if the answer isn't B. Looking at the differences between A and B, A has potential errors in that it:
-Includes the pronoun "it", which could conceivably add some lack-of-clarity; does it refer to "indexing"? (it should) To "the inflation rate"? Maybe even "benefits"? It should go back to "indexing", but the pronoun has at least the potential to cause confusion.
-Again with "it", I could be wrong but "indexing" really should be used as a verb here, so that would mean that we're using a pronoun to refer back to something that is not a noun; if that's true, then A is definitely wrong.
-A is also longer with a redundant "would" (if we're proposing a cause/effect relationship and the cause is conditional, the effect really has to be, as well) and more words overall; if you can't find a flaw with B and A says the same thing, just wordier, you should tend toward the shorter answer.
So like I said, the combination of those three (and #2 may be more airtight now that I think about it) is enough for me to eliminate A, especially because, in comparison, B doesn't have any flaws.
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