Complete each of the following sentences:
a. When a nation has no economic interaction with foreigners and produces everything it consumes, the nation is in a state of __________.
b. According to the law of comparative advantage, each nation should specialize in producing the goods in which it has the lowest __________.
c. The amount of one good that a nation can exchange for one unit of another good is known as the __________.
d. Specializing according to comparative advantage and trading with other nations results in __________.