Concept:Joint supply refers to two or more commodities that are produced together from one raw material or the same production process.
Explanation:Crude oil is refined to produce many products simultaneously, such as kerosene, petrol, diesel, and grease.
This means kerosene and grease are joint products, obtained together from crude oil in the same refining process.
As a result, an increase in the price of crude oil raises the production cost for all these products at once.
Hence, the prices of kerosene and grease rise together, which shows that the two goods are in joint supply.
Competitive supply would not explain this because it involves goods that compete for the same resources rather than being produced together.
Answer:A. Joint supply