A pebble is dropped into a calm pond causing ripples in the form of concentric circles The radius r of the outer ripple is increasing at a constant rate of 1 2 ft sec When the radius is 2 3 ft at what rate is the total area A enclosed by the outer ripple changing O O O O a da dt b dA dt c dA dt d dA dt 6 35 5 52 4 60T ft sec ft sec 3 31 F12 sec ft sec