If two nations have straight-line production possibilities curves,

A) then their trading possibilities curves must lie inside the production possibilities curves.
B) there will be no basis for mutually advantageous trade.
C) there will be a basis for mutually advantageous trade whether the slopes are equal or not.
D) there will be a basis for mutually advantageous trade provided the slopes differ.

If the slopes of the two nations' production possibilities curves differ, then they will have different comparative advantages in producing certain goods. This means that they can specialize in producing the goods in which they have a comparative advantage, and trade with each other for the goods in which they do not have a comparative advantage. This leads to mutually advantageous trade. However, if the slopes of their production possibilities curves are equal, then they have the same opportunity costs of producing goods and there may not be a basis for mutually advantageous trade.