As an Economy Moves Down Along a Straight Line Production

As an economy moves down along a straight line production possibilities frontier,what happens to the opportunity cost of producing the good on the horizontal axis?

A) It remains constant.
B) It decreases.
C) It increases.
D) Above the midpoint it decreases until it equals 1 at the midpoint and then it increases.
E) None of these depicts what happens to opportunity cost.