参考答案：B

解析：

Perfect competition exists when all the firms in the market produce identical products. There is a large number of independent firms, each seller is small relative to the total market, and there are no barriers to entry or exit. Furthermore, each of the many buyers and sellers knows the prices of the competing products in the market. Monopolistic competition is the term used to describe markets where a large number of competitors produce (slightly) differentiated products. Product differentiation gives a degree of market power to firms under monopolistic competition because each firm produces a slightly different product. Laundry detergent and caned spaghetti sauce are examples of products that are sold under monopolistic competition. Oligopoly is a market structure characterized by a small number of producers selling products that may be similar or differentiated. There is interdependence among competitors in that the decisions made by one firm affect the demand, price, and profit of others in the industry. Also, significant barriers to entry exist which often include large economies of scale. The U.S. auto and soft drink industries are examples of oligopoly markets. Monopoly markets are characterized by a single seller of a specific, well-defined product that has no good substitutes. Barriers to entry are high in monopoly markets. Whether a monopoly exists often depends on how we define the product. Microsoft Corp. certainly has a monopoly of sorts on the Windows operating system and related software. If we define the market more broadly, there are other operating systems, word processing programs, spreadsheet programs, and so forth.