Gartland has long been narrowly self-sufficient in both grain and meat. However, as per capita income in Gartland has risen toward the world average, per capita consumption of meat has also risen toward the world average, and it takes several pounds of grain to produce one pound of meat. Therefore, since per capita income continues to rise, whereas domestic grain production will not increase, Gartland will soon have to import either grain or meat or both.
Which of the following is an assumption on which the argument depends?