28.Esther and Karin each own a polluting plant. Esther's plant has a marginal treatment cost = 10T ($10 for the 1^stton, $10(2)=$20 for the 2^ndton...) and Karen's plant = 15T ($15 for the 1^stton, $15(2)=$30 for the 2^ndton...).

Suppose the government plans to charge pollution emission fees to reduce the total pollution by 30 tons. By how many tonnes will Esther have to reduce the level of pollution from his plant with the pollution emission fee?

a. 12

b. 15

c. 18

d. 20