Concept:Equilibrium price is the market-clearing price where quantity demanded equals quantity supplied.
Explanation:At the equilibrium price, buyers want exactly what sellers are willing to supply.
Thus, there is no leftover stock and no unsatisfied buyers.
When quantity demanded equals quantity supplied, excess demand is
0.
A zero excess demand means the market is perfectly cleared.
Therefore, the key condition for market clearing is zero excess demand.
Answer:C. Excess demand is zero.