If the price of commodity X rises and consumers shift to commodity Y, then commodities X and Y are

Step 1: Analyze the question The price of X goes up, and consumers respond by buying Y instead. We need to name the relationship between X and Y.

Step 2: Explain the concept The relationship between two goods depends on how demand for one responds to a change in the price of the other. This is called cross elasticity of demand.

• Substitutes are goods that can replace each other (e.g., Indomie and spaghetti, or Coke and Pepsi). When the price of one rises, demand for the other rises. Cross elasticity is positive.
• Complements are goods used together (e.g., car and fuel, bread and butter). When the price of one rises, demand for the other falls. Cross elasticity is negative.
• Inferior goods are goods whose demand falls when consumer income rises. This is about income, not about the price of another good.

Step 3: Solve

• X becomes more expensive, so consumers buy less of X.
• They move to Y, so demand for Y rises.
• Price of X up and demand for Y up is a positive relationship, which means X and Y are substitutes.

Checking the other options:

• A. Inferior goods relate to income changes, not to a price change in another good. Wrong.
• C. Complements would see demand for Y fall when X's price rises. Wrong.
• D. "Bought together" is just another way of describing complements. Wrong.

Step 4: Answer The correct option is B. substitutes.

Exam tip: Remember "Substitute = Switch." If consumers switch to another good when a price rises, the goods are substitutes. For complements, think "company": they go together, so they rise and fall together in demand.