A runner set out one day to exercise without any specific mileage goal in mind. Using the theories of marginal analysis, what might cause the runner to stop running after just a few short miles?The runner notices a tear forming in an expensive running shoe and wants to patch itThe runner feels great but could be using the time to study economics for the next 10 hours insteadThe runner loves to exercise but it was chilly yesterdayThe runner does not have to put much effort into a three-mile run