I'm not sure why the question is marked incorrect. Shouldn't the net cost be $45? If not, could you please explain?

0H Question 4 Kip and Yale run separate mining companies in the same forest. Both pollute the river flowing through the forest with debris from their work. In the table below, the first row shows the current level of debris that makes its way into the river from their work. The following table set out information that show how much it would cost each company to reduce its pollution by additional increments of 10 pounds. - If a system of tradable permits is introduced, the total cost of cutting current debris in half is Kip Yale Current debris in pounds 60 80 Cost of reducing debris by \"IO pounds $5 $2 Cost of reducing debris by a second 10 pounds $10 $4 Cost of reducing debris by a third 10 pounds $15 $6 Cost of reducing debris by a fourth 10 pounds $20 $3 Cost of reducing debris by a fth \"IO pounds $25 $10 a. $40 \\/ c. $50 d. $55