An externality arises when a firm or person engages in an activity that affects the wellbeing of a third party, yet neither pays nor receives any compensation for that effect. If the impact on the third party is beneficial, it is called a.

Joe pays ann to mow his lawn and ann mows vanna's lawn by mistake. vanna peers out her window and sees ann mowing, yet says nothing to ann about her mistake since vanna needs to have her lawn mowed. when ann approaches vanna for payment, vanna refuses, arguing that she never asked ann to mow her lawn. under these circumstances, ann can recover payment from vanna under: