Last Year Pat Was a Soybean Farmer and Chris Was

Last year Pat was a soybean farmer and Chris was a corn farmer.This year,high demand for ethanol,an automobile fuel made from corn,causes the price of corn to increase.
Refer to the information above.Suppose Pat stopped growing soybeans and began growing corn.What principle would explain that change?

A) The principle of comparative advantage.
B) The scarcity principle.
C) The incentive principle.
D) The equilibrium principle.