Why Does a Nation Experience Increasing Opportunity Cost

Why does a nation experience increasing opportunity cost?

A) As the nation moves from a production point within the PPF to another point also within the PPF, opportunity costs increase.
B) Because the nation cannot produce at the unattainable production points that lie beyond the PPF.
C) As the nation moves from a production point within the PPF to one on the PPF, opportunity costs increase.
D) Resources are not equally productive in producing different kinds of goods and services.
E) When the amount of resources increases, the opportunity cost of all goods and services increases.