Concept:Goods produced together from the same production process are in joint supply.
Explanation:A by-product is a secondary output obtained while producing the main commodity.
For example, producing palm oil also yields palm kernel as a by-product.
Both goods come from one production process, so they are jointly supplied.
Composite supply, on the other hand, means several goods are combined to satisfy one need.
Therefore, X and Y are jointly supplied because X cannot be produced without Y.
Answer:C. jointly supplied