Maria notices that when the price of coffee decreases, she buys more donuts. How would you classify the relationship between coffee and donuts based on this observation?

Maria's observation suggests that coffee and donuts are complementary goods. This means that when the price of coffee goes down, people tend to buy more coffee and, as a result, they also buy more donuts to enjoy with it. The relationship can be explained through cross-price elasticity of demand, which measures how the quantity demanded of one good changes in response to a price change in another good. In this case, since the price of coffee decreases and leads to an increase in donut purchases, we see a negative cross-price elasticity. For example, if coffee becomes cheaper and Maria buys two more donuts for every cup of coffee she buys, this shows how the two items are linked in consumer behavior.