Comparing Volumes of Revolution

Consider the region under $y=\sqrt{x}$ from x = a to 9. Rotating this region about the x-axis yields a volume given by

$V(a)=\pi\left(\frac{81-(a+1)^3}{2}\right)$

(for a proper antiderivative, note that here the effective volume is proportional to $\frac{81-a^2}{2}$, and using the given data, assume the computed volumes for a = 1, 2, 3, and 4 are approximately 40π, 38.5π, 36π, and 32.5π respectively). Rank the intervals [0,1], [0,2], [0,3], [0,4] in order of increasing volume when the lower limit a is varied (interpret [0,a] as indicating the region starting at x=a to x=9).