A 20-ft ladder rests against a vertical wall. Beginning at time t=0 s, the foot of the ladder is pulled horizontally away from the wall so that its horizontal velocity is v(t)=0.4t ft/s. At t=5 s the foot is 12 ft from the wall. What is the instantaneous vertical acceleration of the top of the ladder at t=5 s?
a) −0.55 ft/s2
b) +0.69 ft/s2
c) −0.69 ft/s2
d) −0.31 ft/s2