Answer:
Complement
Step-by-step explanation:
Complementary goods are goods that are consumed together. if the price of a good rises, the quantity of the other good falls and if the price of a good falls, the demand for the complement would rise.
If the price of a cell phone rises, the demand for a protective case shifts to the left, it means that the demand for protective cases fell. so, when the price of cell phones rose, the demand for protective cases fell.