A group of college students throw a party for their graduation. They decide that each of them pick a type of pizza, share all the pizzas and split the total cost evenly. A Tragedy of the Commons problem is likely for each of the following reasons except each pizza is both excludable and rival in consumption. eating the pizzas creates an externality each individual has an incentive to eat as much as possible since no matter how much an individual eats, it would not cost him extra money when one person eats, he may not consider how his choice affects his friends.