When Can Two Countries Gain from Trading Two Goods

Multiple Choice

When can two countries gain from trading two goods?

A) when the first country can only produce the first good and the second country can only produce the second good
B) when the first country can produce both goods,but can only produce the second good at great cost,and the second country can produce both goods,but can only produce the first good at great cost
C) when the first country is better at producing both goods and the second country is worse at producing both goods
D) Two countries could gain from trading two goods under all of the above conditions.