The increase in the demand for a commodity may lead to a decrease in the demand for another if both are

1. Analyze the question The question asks about the relationship between two goods. When demand for one goes up, demand for the other goes down. This is an inverse relationship.

2. Explain the concept

• Complementary demand (joint demand): Goods used together, such as car and fuel, or bread and butter. If demand for one rises, demand for the other also rises.
• Competitive demand: Goods that are substitutes for each other, such as Peak milk and Cowbell milk, or rice and beans. If demand for one rises, people buy less of the other, so its demand falls.
• Composite demand: One commodity with several uses, such as timber (furniture, building, paper) or electricity. Rising demand for one use reduces the supply available for the others, but this describes a single good with many uses, not two goods.
• Same quality: This is not a recognised demand relationship in economics.

3. Solution

• Demand for commodity X increases.
• Consumers switch to X and away from its substitute Y.
• Demand for Y decreases.
• This is the pattern of competitive demand (substitutes).

Checking the options:

• A: Complementary demand would make both demands rise together. Wrong.
• B: "Same quality" has no bearing on the relationship. Wrong.
• C: Composite demand concerns one good with many uses. Wrong.
• D: Competitive demand gives the inverse relationship described. Correct.

4. Answer The correct option is D. in competitive demand.

5. Exam tip Link the word to the behaviour:

• Complementary = Companions (move together)
• Competitive = Compete (one wins, the other loses)
• Composite = Combination of uses (one good, many uses)

When a question says "increase in one leads to a decrease in the other", think substitutes, which means competitive demand.