I was operating on this reasoning as well, but I have now realized the wording of the question evades this flaw. Answer Choice C never refers to fMRI or the subtraction method, but merely states that the areas of the brain which are the most metabolically active will show a rate of oxygen use higher than areas of the brain which are less metabolically active. If it had said that areas which show a higher rate of oxygen use on the fMRI are more metabolically active, then it would have been false, as an area of the brain could show a high differential rate but still be less metabolically active than other regions. Hope this explanation helps in some way.
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I was operating on this reasoning as well, but I have now realized the wording of the question evades this flaw. Answer Choice C never refers to fMRI or the subtraction method, but merely states that the areas of the brain which are the most metabolically active will show a rate of oxygen use higher than areas of the brain which are less metabolically active. If it had said that areas which show a higher rate of oxygen use on the fMRI are more metabolically active, then it would have been false, as an area of the brain could show a high differential rate but still be less metabolically active than other regions. Hope this explanation helps in some way.