# A balloon, which always remains spherical has a variable radius. Find the rate at which its volume is increasing with the radius when the later is $10\; cm$.
$\begin{array}{1 1} (A)\;400\pi\;cm^3/cm \\ (B)\;200\pi\;cm^3/cm \\(C)\;100\pi\;cm^3/cm \\ (D)\;450\pi\;cm^3/cm \end{array}$