What is an​ externality? A. The highest valued alternative that must be given up to engage in an activity. B. The ability of an​ individual, a​ firm, or a country to produce a good or service at a lower opportunity cost than other producers. C. A fixed or variable cost incurred by firms to produce a good or service. D. A benefit or cost that affects someone who is not directly involved in the production or consumption of a good or service. E. The combined benefit and cost to those directly involved in production and consumption of a good or service.