The Production Possibilities Frontier Is Typically Drawn So That It

The production possibilities frontier is typically drawn so that it has a bowed-out shape, or concave to the origin. This shape implies that:

A) it becomes increasingly costly to expand production of one good the more of society's resources are taken from other sectors of the economy.
B) the more of a product is produced, the less costly it becomes.
C) production is characterized by decreasing costs.
D) Both b and c
E) None of the above.