Which of the following statements is incorrect in the theoretical model of Thunen's agricultural location theory?

A、Freight is the only variable that determines the profit.;

B、The result of profit plus freight is a constant.;

C、The result of the subtraction between the market price and the production cost of the same agricultural product is a constant.;

D、The core question is how to increase the product price or reduce the product cost to maximize the profit.