Which of the Following Illustrates the Basic Idea of "Parity

Which of the following illustrates the basic idea of "parity" as a cornerstone of U.S. agricultural policy?

A) The price of corn per bushel should be held constant.
B) A bushel of corn should exchange for more of all goods today than previously because farm incomes have declined.
C) If a bushel of corn exchanged for a pair of pants at a previous time, the same rate of exchange should prevail today.
D) If a bushel of corn exchanged for a pair of pants at a previous time, the rate of exchange today should be two bushels of corn for one pair of pants since productivity in farming has increased faster than in pants manufacturing.