Oligopoly is the predominant market structure in the airline industry. In the express package business, FedEx, UPS, and DHL compete in the global air express industry. In the US, FedEx and UPS are a duopoly. FedEx and UPS financial indicators such as net margin on sales and return on assets are near the average for US companies. What oligopoly model does this suggest and what characteristics of express package business would lead to this outcome.