Seller a Has an Upward-Sloping Supply Curve and Is Willing

Seller A has an upward-sloping supply curve and is willing to supply 400 units of a commodity at a price of $5 per unit. Seller A is now willing to supply 500 units at a price of $5 per unit. Evidently, seller A has experienced a(n) :

A) increase in supply.
B) decrease in supply.
C) increase in quantity supplied.
D) decrease in the quantity supplied.