In which of the following cases is the assumption most reasonable? a. To address the benefits of trade, an economist assumes that there are two people and two goods. b. To estimate the speed at which a beach ball falls, a physicist assumes that it falls in a vacuum. c. To address the impact of taxes on income distribution, an economist assumes that everyone earns the same income. d. To address the impact of money growth on inflation, an economist assumes that money is strictly coins.