The Delivery of First-Class Mail by the U

The delivery of first-class mail by the U.S.Postal Service is an example of

A) a monopoly.
B) perfect competition because consumers have access to other methods of written communication, for example, email and text messaging.
C) monopolistic competition because mail delivery is a differentiated product provided by many firms.
D) an oligopoly because a few other firms provide delivery of letters and packages.