A firm produces volleyballs and soccer balls. What happens to the supply of soccer balls if the market price of volleyballs increases?

a. The opportunity cost of producing soccer balls rises, so the supply curve of soccer balls increases.
b. The opportunity cost of producing soccer balls falls, so the supply curve of soccer balls decreases.
c. The opportunity cost of producing soccer balls rises, so the supply curve of soccer balls decreases.
d. The opportunity cost of producing soccer balls falls, so the supply curve of soccer balls increases.