Concept:Goods are in joint supply when they are obtained together from one production process.
Explanation:If
X is a by-product of
Y, both are produced from the same raw material or process.
This means producing more of
Y automatically yields some amount of
X.
The option that says both are from the same firm is not enough, because a firm can produce unrelated goods.
An increase in
X causing a decrease in
Y describes competitive supply, not joint supply.
The statement that
X and
Y cannot be produced together is the opposite of joint supply.
Therefore, the correct condition is that
X is a by-product of
Y.
Answer:A.
X is a by-product of
Y.