Answer:
B)We have assumed a simple world in which there are only two countries.
Step-by-step explanation:
If the only countries in the world were Daria and Atlantis, then the assumption that all countries benefit from free trade would be correct. When other countries enter into the equation, not all of them may benefit from free trade, for example, if a third country XYZ produced both wheat and fertilizers, a free agreement would not necessarily be beneficial to them.