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.
