The level of potato production in this example will follow the concept of Substitution, letter D. A substitute, or substitute good, in economics and consumer theory is a product or service that a consumer perceives as the same or similar to another product. In the formal language of economics, X and Y are substitutes if the demand for X increases when the price of Y increases.
Potatoes from different farms are an example: if the price of one farm's potatoes goes up, then it can be presumed that fewer people will buy potatoes from that farm and source them from another farm instead.