I am not sure what 8 means, but I can help you with 9.
The formula for the area of a circle in pi r^2, but we do not know what the radius is. We do, however, know that the diameter is 40. The radius is half the diameter, so we can divide 40 by 2 to get the radius. This is equal to 20. Now we plug that into the formula.
pi 20^2 = 400 pi square feet. So the answer is C.