A hot-air balloon rises vertically at a constant rate of 4 m/s. An observer stands on level ground 50 m from the point on the ground directly beneath the balloon. When the balloon is 120 m above the ground, what is the rate of change of the observer’s angle of elevation θ (in rad/s)?
a) 0.0308
b) 0.5408
c) 0.0118
d) 0.0800