$A$ stone is dropped into a quiet lake and waves move in circles at the speed of $ 5 \text{ cm s}^{-1} $. At that instant,when the radius of the circular wave is $ 8 \text{ cm} $,how fast is the enclosed area increasing?

• A
  $ 8 \pi \text{ cm}^2 \text{ s}^{-1} $
• B
  $ 80 \pi \text{ cm}^2 \text{ s}^{-1} $
• C
  $ 6 \pi \text{ cm}^2 \text{ s}^{-1} $
• D
  $ 800 \pi \text{ cm}^2 \text{ s}^{-1} $