A stone is dropped into a quiet lake and waves move in circles at the speed of $5 \;cm/s$. At the instant when the radius of the circular wave is $8 \;cm$, how fast is the enclosed area increasing?

Question

$\begin{array}{1 1} 60\pi\;cm^2/s \\ 80\pi\;cm^2/s \\ 20\pi\;cm^2/s \\ 50\pi\;cm^2/s \end{array} $