A production possibilities curve has "good X" on the horizontal axis and "good Y" on the vertical axis. On this diagram, the opportunity cost of good X, in terms of good Y, is represented by the:

A. distance to the curve from the horizontal axis.
B. distance to the curve from the vertical axis.
C. distance from the origin to the curve.
D. change in Y for each change in X along the curve.

Answer: D