If two items are complementary:
a. the demand for one product is unrelated to the demand of the complementary product.
b. it means that the two products can be substitutes for each other.
c. the costs of their manufacturing and marketing are shared.
d. an increase in the sale of one good causes an increase in demand for the other.
Answer: d. an increase in the sale of one good causes an increase in demand for the other.