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) it makes it easier to illustrate the concepts of scarcity and prices with a bow-shaped curve than it is with 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 will shift outward over time