If b is the mean proportional of a and c, then $(a-b)^{3} : (b-c)^{3}$ is equal to:
[A] $a^{3}:b^{3}$
[B] $a^{2}:c^{2}$
[C] $b^{2}:c^{2}$
[D] $a^{3}:c^{3}$