A stone is dropped into a lake and waves move in circles at a speed of 8 cm/sec. At the instant, when the radius of the circular wave is 10 cm, how fast is the enclosed area increasing?

$120 \pi cm^2/sec$ $140 \pi cm^2/sec$ $80 \pi cm^2/sec$ $160 \pi cm^2/sec$

$160 \pi cm^2/sec$

The correct answer is Option (4) - $160 \pi cm^2/sec$