If the cross-price elasticity of demand between goods X and Y is 1, which of the following must be true?
A) Both goods have unit elastic supply.
B) An increase in the price of good X leads to a decrease in the quantity demanded of good Y.
C) They are substitutes.
D) A percent change in the price of good X is matched by an equal change in the price of good Y.
E) They are both normal goods.