Why is e better answer than c?

Hi @kyoon, thanks for your post. This is an Errors in Reasoning question. Let's evaluate the stimulus first. Premise: a certain scanner will alert the operator whenever a piece of luggage contains an explosive. Premise: the scanner erroneously alerts for only 1% of the pieces of luggage that *contain no explosives.* Conclusion: thus, in 99 out of 100 alerts, explosives *will actually be present* Answer choice (E) is correct because the reasoning substitutes one group (luggage where explosives *are* present, mentioned in the conclusion) for a different group (luggage where explosives are *not* present, mentioned in the second premise) in the statement of a percentage. Answer choice (C) is irrelevant because the conclusion here is about statistics related to the scanner's operation, *not* human operation after the scanner alerts. Hope this helps! Please let us know if you have any additional questions.