# A balloon, which always remains spherical on inflation, is being inflated by pumping in $900$ cubic centimetres of gas per second. Find the rate at which the radius of the balloon increases when the radius is $15\; cm$.
$\begin{array}{1 1} (A)\;\frac{-1}{\pi}cm/s \\ (B)\;\frac{-1}{\pi}m^2/s \\ (C)\;\frac{-1}{\pi}m/s \\(D)\;\frac{1}{\pi}cm/s \end{array}$