Answer:
C. two countries should benefit from trade unless both have equal opportunity costs in every good
Explanation:
When a country have a comparative advantage, its cost of producing a specific product will be cheaper compared to another country.
Let's say that there are two countries.
Country A and Country B. Country A able to produce rice in a really low cost (opportunity cost for producing other products beside rice is higher) . Country B able to produce meat in a really low cost. (opportunity cost to product other products beside meat is higher)
When the two countries trade with one another, they'd both will be benefited. Rather than producing its own rice, it will be cheaper for country B to purchase rice from country A and vice versa.