Answer (D) is correct. The oligopoly model is much less specific than the other market structures, but there are typically few firms in the industry. Thus, the decisions of rival firms do not go unnoticed. Products can be either differentiated or standardized. Prices tend to be rigid (sticky) because of the interdependence among firms. Entry is difficult because of either natural or created barriers. Price leadership is typical in oligopolistic industries. Under price leadership, price changes are announced first by a major firm. Once the industry leader has spoken, other firms in the industry match the price charged by the leader. The mutual interdependence of the firms influences both pricing and output decisions.