One of the differences between perfect competition and oligopoly is: A) that an oligopolist rarely earns economic profit, whereas a perfectly competitive firm will always earn economic profit. B) that while a perfectly competitive firm earns zero economic profit in the long run, the oligopolist can earn positive economic profit in the long run. C) that a perfectly competitive firm will advertise while an oligopolist will not advertise. D) that perfectly competitive firms often engage in non-price competition, whereas oligopolists don't.