Passage Summary: Because the city ought to do anything that makes economic sense, and a nice subway system makes economic sense, the city must buy new cars for the trains.

Conclusion: The city is required to purchase new subway cars.

Reasoning: The city should always do what is economically sensible, and maintaining an efficient, attractive subway system is economically sensible.

Analysis: We have a clear gap between the general goal (an efficient, attractive subway) and the specific action (buying new cars). The premises tell us the city should pursue the goal, but they don't explicitly state that buying new cars is the only way to achieve it. To bridge this gap, we need an assumption that connects the goal directly to the purchase. Look for an answer that guarantees that if the city needs an efficient and attractive system, then buying new cars is a necessary or sufficient step to get there.