Which of the Following Describes a Barrier to Entry

Which of the following describes a barrier to entry?

A) something that establishes a barrier to expanding output
B) anything that protects a firm from the arrival of new competitors
C) a government regulation that bars a monopoly from earning an economic profit
D) firms already in the market incurring economic losses so that no new firm wants to enter the market
E) Firms are legally prohibited from exiting the market in order to enter another market.