Gilberto needs to purchase baseballs and bats for his youth baseball league. He'll need at least 15 pieces of equipment, but he only has $435 to spend. Baseballs cost $20 each and bats cost $35 each. If Gilberto wants to purchase the most amount of equipment and still stay within his budget, which combination of baseballs and bats is optimal? (2 points)
a (20, 1)
b (13, 5)
c (13, 2)
d (10, 5)