This lecture continues the discussion on market regulation, focusing on the implications of external costs in production. The instructor explains how production often incurs costs that are not accounted for by producers, leading to inefficiencies in market outcomes. The lecture illustrates the concept of social optimum, where the quantity produced aligns with the overall social costs and benefits. The instructor discusses the role of taxes and subsidies in correcting market failures, emphasizing how a tax can adjust the supply curve to reflect external costs. The lecture also explores the idea of subsidies for not producing, particularly in agricultural contexts, and how these can incentivize producers to reduce output. The instructor further elaborates on the concept of tradable emission permits as a mechanism to cap production while allowing flexibility among producers. The discussion highlights the importance of aligning private incentives with social welfare to achieve efficient market outcomes.