All resources are scarce so choices must be made among a limited set of possibilities, so a decision to have more of one thing is a decision to have less of something else. The cost of any decision is its opportunity cost the value of the next best alternative that is given up. Forces of supply and demand determine these decisions. To get more wheat the farmer must move resources out of soybean production and into wheat production. From B to C, an additional reduction in 10,000 bushels of soybeans yields only 14,000 bushels of wheat.
Inputs tend to be specialized.
Some land may be better suited for wheat versus soybean production. As the production of one good expands, the opportunity cost of producing another unit of this good generally increases. In this case, the PPF is bowed outward. Underlying this principle is the notion that inputs tend to be specialized. If they were not, then the PPF is a straight line.