Concept:The reason for international trade is the difference in comparative cost between nations.Explanation:According to the theory of comparative cost, trade takes place when the ratios of producing goods differ between two countries.Each country specialises in producing the commodity in which it has a comparative advantage.A country has a comparative advantage when it can produce a good at a lower opportunity cost than other countries.Hence, it is this difference in comparative cost, not absolute cost, that makes international trade beneficial.Answer:B. comparative cost