2There are four basic types of competition in business that form a continuum from pure competition through monopolistic competition and oligopoly (商品应垄断) to monopo- ly. At one end of the continuum, pure competition results when every company has a simi lar product. Companies that deal in commodities such as wheat or corn are often involved in pure competition. In pure competition, it is often the ease and efficiency of distribution that influences purchase.In contrast, in monopolistic competition, several companies may compete for the sale of items that may be substituted. The classic example of monopolistic competition is coffee and tea. If the price of one is perceived as too high, consumers may begin to purchase the other. Coupons and other discounts (折扣) are often used as part of a marketing strategy to influence sales.Oligopoly occurs when a few companies dominate the sales of a product or service. For example, only five airline carriers control more than 70 percent of all ticket sales in the United States. In oligopoly, serious competition is not considered desirable because it would result in reduced revenue for every company in the group. Although price wars do occur, in which all companies offer substantial savings to customers, a somewhat similar tendency to raise prices simultaneously is also usual.Finally, monopoly occurs when only one firm sells the product. Some monopolies have been tolerated for producers of goods and services that have been considered basic or essen tial, including electricity and water. In these cases, it is government control, rather than competition, that protects and influences sales. The word "it" in Paragraph 3 refers to （）.

A. a product or service
B. competition
C. revenue
D. oligopoly