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

According to William Shepherd's examination of competitive trends in the U.S.economy, a tight oligopoly

A) is a single firm that controls the entire market and can block entry
B) is an industry in which the top four firms supply more than 60 percent of the market, have stable market shares, and cooperate with each other
C) is an industry in which the top four firms supply more than 60 percent of the market, have unstable market shares, and do not cooperate with each other
D) is an industry in which a single firm has over half the market share and no close rival
E) is an industry in which a single firm has over one-third of the entire market, the market share is stable, and the firm cooperates with other firms in the industry