What is externality? a. a side effect of a good service generating benefits or costs to someone who doesn't decide how much to produce or consume b. someone who would choose to pay for a certain good or service but does not get the benefits of it if provided as a public good c. someone who wouldn't choose to pay for a certain good or service but who'd get the benefits of it anyway if provided as a public good d. a shared good or service for which it would be impractical to make consumers pay individually and to exclude nonpayers