Two goods are said to be complements if they are generally or necessarily consumed together, such as tea and sugar cubes, DVD players and DVD discs, petrol and car, or printers and ink cartridges. In addition, there is a negative relationship between the price on one and the demand for the other. To put it more clearly, if there is a rise in the price of one of the products, the demand for the other product will decrease. The correct answer is C.