A 25-ft ladder leans against a vertical wall, forming an angle θ (in radians) with the ground. The foot of the ladder is sliding away from the wall at a rate of 2ft/s. What is dtdθ when the foot of the ladder is 7ft from the wall?
a) 121rad/s
b) −121rad/s
c) −61rad/s
d) −247rad/s