Answer: Tea and Coffee
Notes: Complementary goods, defined by their negative cross elasticity of demand, are products whose demand is influenced by one another. When the price of a good declines, the demand for its complementary good rises. Printers need ink cartridges, tea is often consumed with sugar, and mobile phones require SIM cards for operation. However, tea and coffee are substitutes, not complements, as they are consumed separately and often in place of one another, hence they are not complementary goods.
