If a person chooses to walk rather than drive, there is one less vehicle emitting pollution into the air than there would be otherwise. Ttherefore if people would walk whenever it is feasible for them to do so, then pollution will be greatly reduced. Which one of the following is an assumption on which the argument depends? ['There are people who never drive but who often walk.', 'People sometimes drive when it is feasible to walk instead.', 'Cutting down on pollution can be achieved in a variety of ways.', 'Taking public transportation rather than driving is not always feasible.']