Concept:Joint demand refers to goods that are consumed together as complements.
Their demand moves in the same direction.
Explanation:When two goods are used together, they are said to be in joint demand.
Examples include coffee and sugar, or cars and petrol.
If the price of one commodity rises, consumers will buy less of it.
Consequently, the demand for the other complementary commodity also falls.
This matches exactly what the question describes.
Competitive demand would apply to substitutes, where an increase in the price of one good raises the demand for the other.
Composite demand involves a single good used for several purposes.
Derived demand relates to factors of production needed to make a final product.
None of these fits the situation in the question.
Answer:B. joint