GMAT Critical Reasoning (CR) | Re: Preliminary tests show that the drug known as Mephaline

I don't understand how b is the answer.

Argument concludes that there must be something qualitatively different about these two types of memory loss. the author arrives at this conclusion because of difference in applicability of drug in both kind of memory loss. Author stated in the premise that Mephaline did not appear to slow or reverse memory loss in people who experienced memory loss due to a brain injury.

SO, argument basically assumes that difference in cause/origin of both kind of memory
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