A market maker faces the following demand and supply for widgets. Eleven buyers are willing to buy at the following prices: $15, $14, $13, $12, $11, $10, $9, $8, $7, $6, $5. Eleven sellers are also willing to sell at the same prices. If the market maker is free to choose the number of transactions he can make, what is his maximum profit?

a.$8

b.$10

c.$18

d.$28