From least integrated to most integrated, the levels of economic integration are a: （ ） A. common market, a free trade area, an economic union, a customs union, and a political union. B. free trade area, a customs union, a common market, an economic union, and a political union. C. customs union, a free trade area, a common market, a political union, and an economic union. D. common market, an economic union, a customs union, a free trade area, and a political union.