At the fishing hole, people come from all around to catch fish to sell at the fish market. The total number of fish caught is F=10x-(x^2) where x is the number of fishermen. Suppose it costs each person $20 a day to fish and that fish sell for $10 each at the market. At the social optimum, how much would it hurt all the other fishermen (combined) if one more person started fishing?

a) $30

b) $20

c) $10

d) $40