题目4单选题
When an industry has many firms, the industry isA. an oligopoly if the firms sell differentiated products, but it is monopolistically competitive if the firms sell identical productsB. an oligopoly if the firms sell differentiated products, but it is perfectly competitive if the firms sell identical productsC. monopolistically competitive if the firms sell differentiated products, but it is perfectly competitive if the firms sell identical productsD. perfectly competitive if the firms sell differentiated products, but it is monopolistically competitive if the firms sell identical products