Concept:Comparative advantage means producing a good at a lower opportunity cost than other countries.Explanation:A country should specialize in the goods it can produce at the lowest opportunity cost.It should then trade with other countries for goods that are more expensive for it to produce.This raises total output and benefits all trading partners.Therefore, the principle encourages trade when a country can produce a commodity at a lower cost than others.Answer:B. engage in trade if it can produce a commodity at a lower cost.