Answer:
The answer is: a change in the price at which a substitute good is sold
Explanation:
A shift in supply means a change in the quantity supplied at every price.
Let's assume we sell product A. If the price of a substitute product B increases, then the quantity demanded for product A will increase as the quantity demanded for product B decreases. That will cause an increase in the quantity supplied of product A, which may in turn rise the price of product A until again both products (A and B) match their prices.
Instead, a shift in the supply curve means that the quantity supplied of a product will change at every price level.
