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

According to William Shepherd's examination of competitive trends in the U.S.economy,if the top four firms supply more than half of market output and the market share of each firm is stable,the industry _____

A) is usually a pure monopoly.
B) is a dominant firm.
C) is a tight oligopoly.
D) is effective competition.
E) is monopolistic competition.