Why Can't an Economist Say for Certain That a Monopolistic

Why can't an economist say for certain that a monopolistic competitive firm will always earn zero economic profits in the long run?

A) Barriers to entry exist.
B) The very large number of buyers indicates that there will always be demand for the firm's product.
C) The firms in the industry do not produce identical products.
D) The firms practice price competition, so at least some firms will always be charging a lower price than other firms and will sell more as a result.
E) The firms face a horizontal demand curve.