Final answer:
If all firms in an industry acted as a single seller, they could become a cartel and maximize profits, but such collusion is often illegal in many regions.
Step-by-step explanation:
If all firms in an industry joined together and behaved as a single seller, they could maximize their joint profits. We refer to such a group as a cartel. However, it's important to note that in many regions, such as the European Union and the United States, it is illegal for firms to engage in such behavior because it restricts competition and is considered a form of collusion. Instead, in perfectly competitive markets, where profit-maximizing firms and utility-maximizing consumers interact, the market often reaches a state of both productive and allocative efficiency.