Suppose that eight workers can manufacture 70 radios per day and that nine workers can manufacture 90 radios per day. If radios can be sold for $20 each, the value of marginal product of the ninth worker is a. $200. b. 20 radios. c. 90 radios. d. $400.

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: