Which Statement Best Describes the Opportunity Cost Evident in the Production

Which statement best describes the opportunity cost evident in the production possibilities frontier (PPF) for the accompanying figure?

A) The law of increasing relative cost applies because the PPF is a straight line.
B) The law of increasing relative cost applies because the PPF is bowed outward.
C) The opportunity cost is constant because the PPF is a straight line.
D) The opportunity cost is constant because the PPF is bowed outward.
E) The opportunity cost decreases because the line has negative slope.