A is a demand theory, D is a manufacturer theory, C is a production theory, and only B is a trade theory. Factor endowment theory emphasizes the decisive role of different factor endowments and different production functions of different commodities in different countries and regions on trade. A country produces and exports a large number of products that use its rich elements, and imports a large number of products that use its scarce elements. The reason for the trade between the two countries is that the resource intensity of the two countries is different, so it belongs to inter-industry trade.