A 10-ft ladder leans against a vertical wall. When the foot of the ladder is 6 ft from the wall and sliding away at 2 ft/s, what is the instantaneous rate of change of the height of the top of the ladder?
a) −0.75 ft/s
b) +1.5 ft/s
c) −1.5 ft/s
d) −2.67 ft/s