GMAT Sentence Correction (SC) | Re: Rising inventories, when unaccompanied correspondingly by

I got the right answer. Is the use of 'if' in this sentence correct? If there was another sentence with 'whether', will we pick that one over this? I am so confused because GMAT SC rules are very subjective. Once before, I read if cannot be used, but now we are using if and that's the correct answer now all of a sudden. It's mainly because of these various GMAT tutorials which don't pay much attention to explain the exception to rules.

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