“So as to” vs. “so that” is a “red-herring” split here. In other words, either form is fine. The GMAT has gone back and forth on "so as to" (see Ch. 9 of our SC book), but you wouldn't want to eliminate on this basis alone.
In any case, the idiom “so as to” will generally have an intervening term: “So X as to Y.”
The award-winning pumpkin was so large as to require a special scale.
Note that the odd form of this idiom puts "require" in the present tense. Its size was large enough to cause this requirement.
We can also use “so that” with a more straightforward construction:
The award-winning pumpkin was so large that it required a special scale.
In the case of the posted problem, the difficulty with B has nothing to do with "so as to" vs. "so that"--it's the word “can.” We want to say that the changes actually are indistinguishable, not that they *can* be indistinguishable. This is a great example of the kind of meaning-based difference that can catch you if you are only focused on grammar rules. Beware!
Dmitry Farber | Manhattan GMAT Instructor | New York
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