The law of supply states that, other things being constant, as price increases

1. Analyze the question The question asks what the law of supply says happens when price rises, with other factors held constant (ceteris paribus).

2. Explain the concept The law of supply says there is a direct (positive) relationship between price and quantity supplied. When the price of a good rises, producers are willing and able to offer more of it for sale, because higher prices mean higher profit.

Note the difference between these two terms, which JAMB often tests:

• Change in quantity supplied: caused only by a change in the good's own price. It is shown by a movement along the same supply curve.
• Change in supply: caused by non-price factors such as cost of production, technology, taxes, subsidies, number of sellers, weather, and prices of related goods. It is shown by a shift of the whole supply curve.

3. Solution

• Price is the only factor that changes here. All other factors are constant.
• A price change therefore causes a movement along the supply curve, which is a change in quantity supplied.
• The direction is a rise, because the relationship is direct.

Checking the options:

• A. Supply increases: wrong, because this would need a shift of the curve caused by non-price factors.
• B. Supply decreases: wrong, for the same reason, and the direction is also wrong.
• C. Quantity supplied increases: correct.
• D. Quantity supplied decreases: wrong, because the direction is opposite to the law.

4. Answer C. Quantity supplied increases.

5. Exam tip Remember this rule: "Price changes → quantity moves. Other factors change → the whole curve shifts." If the question mentions the good's own price, the answer will use the words "quantity supplied" or "quantity demanded", never "supply" or "demand" alone.