According to William Shepherd's Examination of Competitive Trends in the U.S.economy,a

According to William Shepherd's examination of competitive trends in the U.S.economy,a single firm which controls the entire market and is able to block entry _____

A) is usually a pure monopoly.
B) is a firm with over half the market share and no close rival.
C) is one of four firms that together supply more than 60 percent of the market.
D) is a firm that sells a product with many close substitutes.
E) is one of four firms that work together to block entry into the market.