At an instant when the foot of a 20-ft ladder is 12 ft from a vertical wall, the foot is sliding away from the wall at a rate of 3 ft/s. What is the instantaneous velocity of the top of the ladder at that moment?
a) −1.80 ft/s
b) 2.25 ft/s
c) −2.25 ft/s
d) −4.00 ft/s