A 13-ft ladder rests against a vertical wall. The top of the ladder is sliding down the wall at a constant rate of 2 ft/s. At the instant when the top of the ladder is 5 ft above the ground, how fast is the foot of the ladder moving away from the wall?
a) 524 ft/s
b) −65 ft/s
c) 65 ft/s
d) 45 ft/s