An airplane is flying such that its altitude h and horizontal distance x (both in meters) from a radar station satisfy θ=arctan(xh). At a certain instant h=1000, x=3000, dtdh=150m/s, and dtdθ=0.020rad/s. What is the airplane’s horizontal speed dtdx at that instant?
a) 270m/s
b) 250m/s
c) 450m/s
d) 260m/s