A rectangle is inscribed in the region bounded by the x-axis and the graph of y=9−x2 in the first quadrant so that one side of the rectangle lies on the x-axis and the upper vertices lie on the curve. Let (x,9−x2) be the upper-right vertex of the rectangle. For which value of x is the area of the rectangle greatest?
a) 3
b) 2
c) 2
d) 23