Final answer:
When the price of good X falls and the demand for good Y decreases, we can conclude that X and Y are substitute goods.
Step-by-step explanation:
When the price of good X falls and the demand for good Y decreases, we can conclude that X and Y are substitutes. Substitute goods have positive cross-price elasticities of demand, which means that a higher price for one good leads to greater consumption of the substitute good. In this case, the decrease in demand for good Y indicates that consumers are substituting it with good X due to the lower price.