When a change in the demand for commodity A leads to a change in demand for commodity B in the same direction, demand for A & B is

Step 1: Analyze the question The question asks what kind of demand relationship exists when a change in demand for commodity A causes demand for commodity B to move in the same direction (both rise or both fall).

Step 2: Explain the concepts

• Competitive demand (substitutes): Goods that can replace each other, such as Indomie and spaghetti. If demand for A rises, demand for B falls, so they move in opposite directions.
• Complementary demand (joint demand): Goods used together, such as cars and petrol, or bread and butter. If demand for A rises, demand for B also rises, so they move in the same direction.
• Composite demand: One commodity with several uses, such as wood for furniture, fuel and building. Demand for it is the sum of the demands for all those uses.
• Independent demand: Goods with no relationship. A change in demand for one has no effect on the other.

Step 3: Apply to the question

• A rise in demand for cars leads to a rise in demand for petrol, and a fall in demand for cars leads to a fall in demand for petrol.
• This is movement in the same direction, which matches complementary demand.

Step 4: Answer The correct option is B. complementary demand.

Tip: Think of "complement" as "completes." Goods that complete each other are bought together, so their demand rises and falls together. For substitutes, remember "opposite directions" (if the price of rice rises, people switch to beans).