The Reason That the Production Possibilities Frontier Is Usually a Bow-Shaped

The reason that the production possibilities frontier is usually a bow-shaped curve instead of a straight line is that _____

A) a curve makes it easier to illustrate the concepts of scarcity and prices than does a straight line.
B) early economists began drawing them in this way and the convention has continued throughout the years.
C) output eventually reaches a maximum and then declines.
D) resources are not perfectly adaptable to the production of all goods.
E) the frontier shifts outward over time.