The angle of elevation of a jet plane from a point A on the ground is $60^{\circ}$ After a flight of $30$ seconds , the angle of elevation changes to $30^{\circ}$. If the jet plane is flying at a constant height of $3600\sqrt 3\;m$, then the speeds of the jet plane is _______

Question

$\begin{array}{1 1}(A) \;800\;km/hr \\(B)\; 850\;km/hr \\(C)\;864\;km/hr \\(D)\;854\;km/hr \end{array}$