Which of the following statements is true?

a. Specialization and trade along the lines of comparative advantage allows nations to consume more than if they were to produce just for themselves.

b. Free trade theory suggests that when trade takes place any gains made by one nation comes at the expense of another.

c. According to the theory of comparative advantage, a nation should specialize in the production of those goods for which it has an absolute advantage.

d. All of the above.