Refer to the Figure Shown,which Represents the Production Possibilities Frontiers

Refer to the figure shown,which represents the production possibilities frontiers for Countries A and B.One of the reasons why Country A and Country B are not realistic representations of actual countries is:

A) the production possibilities curves are straight lines; realistic ones would be concave.
B) they only represent the production of two goods.
C) they only represent two countries.
D) All of these statements are true.