We see quite a bit of international trade in the real world. And trade is driven by specialization. So why don’t we see full specialization—for instance, all cars in the world being made in South Korea, or all the mobile phones in the world being made in China? Choose the best answer from among the following choices. 

a. High tariffs.

b. Extensive import quotas.

c. Increasing opportunity costs.

d. Increasing returns.