Could you please explain why E is a better answer than B?

Is it because the system in question refers to "the catalog" mentioned in the first sentence? And seeing as there is no evidence to support that the catalog was defective, the system has not failed, as B describes?

@JeremyG, Happy to help. You asked why (E) is correct and (B) is wrong, so let's take a look. (B) says, "A deficiency in a system is isolated by arguing that the system failed to control one of the objects that it was intended to control." The problem with (B) is that the stimulus does not appear to be making the argument that the library's organization system is deficient or at fault. Your line of thinking is correct. (E) says, "The conclusion is supported by ruling out other possible explanations of an observed fact." The observed fact that (E) mentions is that the book Horatio wants isn't in its place on the bookshelves. The argument rules out other potential locations of the book to arrive at its conclusion, so this is the correct answer choice. Does this answer your question? Let us know if you have any other questions!

Yes, thank you!

@JeremyG, great! Let us know if you have any other questions—we're here to help!