New subway cars are financially affordable. Cost-effective subway cars are an integral part of an efficient subway system. New subway cars are required in order for the city to have a subway system that is efficient and attractive. Investment in new subway cars makes better economic sense than many of the other investment options open to the city.
Having an efficient, attractive subway system makes good economic sense. So, the city needs to purchase new subway cars, since the city should always do what makes good economic sense.
The conclusion drawn above follows logically if which one of the following is assumed?