The correct answer is D. substitutes.
Substitutes are goods that can be used in place of each other. When the price of one substitute good decreases, the demand for the other substitute good will decrease. For example, if the price of coffee decreases, the demand for tea will decrease. This is because people will be more likely to buy coffee instead of tea.
Complements are goods that are used together. When the price of one complement good decreases, the demand for the other complement good will increase. For example, if the price of cars decreases, the demand for gasoline will increase. This is because people will be more likely to buy cars if they are less expensive, and they will need gasoline to drive their cars.
Normal goods are goods that people demand more of when their income increases. Inferior goods are goods that people demand less of when their income increases.
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