Train service suffers when a railroad combines commuter and freight service. By dividing its attention between its freight and commuter customers, a railroad serves neither particularly well. Therefore, if a railroad is going to be a successful business, then it must concentrate exclusively on one of these two markets.
For the argument to be logically correct, it must make which one of the following assumptions?
[A]. Commuter and freight service have little in common with each other.
[B]. The first priority of a railroad is to be a successful business.
[C]. Unless a railroad serves its customers well, it will not be a successful business.
[D]. If a railroad concentrates on commuter service, it will be a successful business.
[E]. Railroad commuters rarely want freight service as well.
Maybe you saw the problem with this logic right away: The evidence deals with train service and customer satisfaction, while the conclusion jumps to the concept of railroads as “a successful business.” Are these things the same? No—you probably know the style of the testmakers better by now. As you’ve most likely noticed, they’re very particular about the way they use words on the CAT. Here, the scope shifts from the first sentence to the last. Even though “train service” and “successful business” seem correlated in some way, they’re not the same thing. The author should have explicitly stated that there’s a connection between the two; without such a stated connection, the argument falls flat. It seems like an obvious connection, but it’s nonetheless necessary for the argument to work. By relating service and customer satisfaction to the success of the railroad business, choice (C) bridges this narrow gap in the argument. (C) is thus the assumption on which the argument relies.
(A) For all we know, these two types of train service may be closely related. (A) seems to lend support to the assertion in the first sentence, but why the railroads can’t run both services at once successfully isn’t discussed and isn’t relevant to the argument as a whole, so (A) need not be assumed.
(B) Prioritization isn’t an issue. All the author does is posit a condition that’s necessary if a railroad is to be a successful business. Thus, the author need not assume that being a successful business is the railroad’s first priority; this argument could work even if being a successful business was far down on the list of a railroad’s priorities.
(D), far from a necessary assumption, is an unwarranted inference based on the final sentence. The last sentence expresses the concept of necessity (it’s necessary to concentrate on one service to be successful), while (D) interprets this as a statement of sufficiency (all the railroad needs to do is concentrate on one service to be successful).
(E) goes astray by trying to mix the two service lines, but commuters’ demand for freight service is an irrelevant issue that in no way plugs the hole in this argument.
Scope shifts cover a lot of ground. Here’s an example where the author shifts the scope of the argument by introducing a new word or phrase, in this case, “successful business.” If you noticed the shift from “train service” in the evidence to “successful business” in the conclusion, you probably had no problem picking out the assumption in (C) that bridges the gap between these two distinct elements of the argument.
Don’t do the author’s work for her! You may have missed the distinction above if you inferred the connection in choice (C) on your own. Sure, it’s common sense that if you don’t serve customers well, you probably won’t have a successful business. Regardless, making this judgment still entails taking a logical leap, because this author never makes this connection that holds the argument together. It’s the author’s job to present as tight an argument as possible, and that includes tying up the loose ends. If (C) weren’t true (the Denial Test)—and it’s possible theoretically for railroads to succeed without serving customers particularly well—then this argument would fall apart.