Suppose the economy initially produces 12,000 gallons of drinking water and 500 million tons of coal, which is represented by point A. The opportunity cost of producing an additional 4,000 gallons of drinking water (that is, moving production to point B) is tons of coal. Suppose, instead, that the economy currently produces 420 million tons of coal and 16,000 gallons of drinking water, which is represented by point B. Now the opportunity cost of producing an additional 4,000 gallons of drinking water (that is, moving to point C) is tons of coal.
Comparing your answers in the two previous paragraphs, you can see that the opportunity cost of 4,000 additional gallons of drinking water at point B is the opportunity cost of 4,000 additional gallons of drinking water at point A. This reflects the .