Train service suffers when a railroad combines commuter and freight service.
Conclusion (therefore)
A railroad must concentrate exclusively on either commuter or freight service in order to be successful.
Evidence
The railroad suffers when it combines commuter and freight service because it divides its attention between the two markets.
This division means that the railroad serves neither market particularly well.
Evaluate
Pattern: This argument relies on a causal chain: dividing attention leads to poor service, which in turn prevents business success. In order for the conclusion—that exclusive concentration is necessary for success—to hold, the argument must assume that serving customers well is essential to being a successful business.
Goal
We need an answer that bridges the gap between poor customer service and business success. That is, the argument assumes that unless a railroad serves its customers well, it will not be successful. The correct assumption would indicate that high-quality service is a precondition for business success, meaning that divided attention (and the resulting poor service) is indeed detrimental to success.
Too Strong / Irrelevant Comparison
Answer Choice (A) suggests that commuter and freight service have little in common. Although the two markets are different, the argument does not require that they have “little in common” in order for divided attention to be harmful to service quality.
Out of Scope
Answer Choice (B) claims that the first priority of a railroad is to be a successful business. The argument discusses what is necessary to achieve success, not the railroad’s priorities, so this isn’t a needed assumption for the reasoning.