A heavy industrial plant located in the center of a small town emits particulate matter into the atmosphere. Suppose that the concentration of fine particulate matter (in parts per million) at a point d miles from the plant is given by

C = 65 - 6d²

If the boundaries of the town form a square 4 miles long and 4 miles wide, what is the average concentration of fine particulate matter throughout the town? Express C as a function of x and y, and set up a double integral and evaluate it.