In which of the following cases is the assumption most reasonable ?

A. To address the impact to taxes on income distribution an economist assumes that everyone earns the same income.
B. To address the impact of money growth on inflation, an economist assumes that money is strictly coins.
C. To model the benefits of trade. an economist assumes that there are two people and two goods
D. To estimate the speed at which a beach ball falls, a physicist assumes that if falls in a vacuum.