What are the four most important ways a firm becomes a​ monopoly? The four main reasons a firm becomes a monopoly​ are:

A. the government blocks​ entry, control of a key​ resource, network​ externalities, and economies of scale.
B. antitrust​ legislation, control of a key​ resource, no close​ substitutes, and economies of scale.
C. the government blocks​ entry, control of a key​ resource, network​ externalities, and diseconomies of scale.
D. antitrust​ legislation, control of a key​ resource, arbitrage, and economies of scale.
E. the lack of patents and​ copyrights, control of a key​ resource, network​ externalities, and economies of scale.