The best answer of the choices provided here is B.

According to economic theory, countries should produce and export those goods for which they have a comparative advantage.  One country (Country A) has a comparative advantage over another country (Country B) in a given good when Country A produces that good at a lower opportunity cost than Country B does.  If every country produces and exports the things that it can make at a low opportunity cost, then total production worldwide rises and, in theory, all countries are better off.  Therefore, B is the correct answer.