China has, for centuries, subsisted on a diet consisting largely of grains like rice. However, as income levels in China rise, demand for protein based foods has increased. It takes several kilograms of rice grains to produce one kilogram of meat. Therefore, since income levels continue to rise and viable agricultural space for rice production is limited, China will soon have to import more of its food supplies in order to feed the country.
Which of the following is an assumption on which the argument depends?