Dent 'N' Scratch Used Cars and Trucks Employs 3 Salesmen

Dent 'n' Scratch Used Cars and Trucks employs 3 salesmen. Data for their sales last month are shown in this table: $\begin{array}{l|r|r|r}\hline & \text { Cars Sold } & \text { Trucks Sold } \\\hline \text { Larry } & 10 & 5 \\\hline \text { Joe } & 9 & 9 \\\hline \text { Ralph } & 3 & 12 \\\hline\end{array}$  
Based on last month's data, Joe's opportunity cost of selling a truck is selling:

A) 9 cars.
B) 1 car.
C) 4 cars.
D) 1/3 of a car.