Answer:
If the fall in price of good Y causes demand for good X to fall as well, the goods Y and X are complementary.
Explanation:
Complementary goods are those goods that people tend to buy together, since, as the word implies, they fulfill complementary purposes.
An example of two complementary goods are BBQ Sauce and Meat.
Meat is often used to grill/roast, and it is accompanied with BBQ Sauce. If BBQ Sauce price goes down, it is likely that more of it will be bought as well as more meat. The opposite occurs if the price of either good goes up. Hence, both goods are complementary.
