The production possibilities frontier shows all efficient combinations of outputs.Points B,C,and D are taken from comparable(比较的)points on the production curve. The production possibilities frontier is concave(凹面的) because its slope(the marginal rate of transformation)increases as the level of production of food increases. The production possibilities frontier shows all efficient combinations of outputs. Points B, C, and D are taken from comparable (比较的) points on the production curve. The production possibilities frontier is concave (凹面的) because its slope (the marginal rate of transformation) increases as the level of production of food increases