Each country will select the work item which when compared to the other items. Country can be manufactured with a minimum cost and exchange with another country for two countries it would have been useful.
Each country will choose its production, compared to other products. His country can produce with the lowest costs. To exchange with another country, the two countries will benefit.
Each country will choose to produce, which compared with other goods. His country can be produced at low cost. Then the exchange with another country and country are 2 will benefit.