The Slope of a Typical Production Possibilities Frontier Reflects the Fact

The slope of a typical production possibilities frontier reflects the fact that

A) some systems of market organization are more efficient than others.
B) the invisible hand always functions smoothly in a market system without government intervention.
C) when resources are allocated efficiently, it's impossible to produce more of anything without producing less of something else.
D) production is only possible when resources are allocated efficiently.