A Farmer Puts Pesticides on His Crops, Which Then Run

A farmer puts pesticides on his crops, which then run off into a nearby river. The pesticides are removed when the water is treated for residential consumption. The table that follows shows the costs of and benefits from the use of pesticides. In the table, MB is the marginal benefit to the farmer, MPC is the marginal private cost to the farmer of buying and spreading the pesticide, and MEC is the marginal external cost of treating the water. How much pesticide will the farmer use? What is the socially optimal amount of pesticide? Why is it not economically efficient to cease all pollution? Use a graph to support your response.
$\begin{array}{|c|c|c|c|}\hline \begin{array}{c}\text { Pesticide } \\\text { (lbs) }\end{array} & \boldsymbol{M B} & \boldsymbol{M P C} & \text { MPC } \\\hline 10 & \$ 325 & \$ 20 & \$ 30 \\\hline 20 & \$ 300 & \$ 40 & \$ 60 \\\hline 30 & \$ 250 & \$ 60 & \$ 90 \\\hline 40 & \$ 200 & \$ 80 & \$ 120 \\\hline 50 & \$ 100 & \$ 100 & \$ 150 \\\hline 60 & \$ 50 & \$ 120 & \$ 180 \\\hline\end{array}$