In some countries, national planners have attempted to address the problems resulting from increasing urbanization by reducing migration from rural areas. But some economists have suggested an alternative approach. These economists assert that planners could solve these problems effectively by trading goods or services produced by a predominantly urban population in order to obtain the agricultural products that were previously produced domestically. Which one of the following, if true, would provide the most support for the economists' assertion? ['Government subsidies to urban manufacturers can ease the problems caused by the migration of people from rural to urban areas.', 'Problems associated with migration to cities from rural areas are primarily due to trade imbalances between countries.', 'A scarcity of agricultural products is a central element of many problems created by urbanization.', 'All problems that have economic causes must have economic solutions.']