The height of the mountain if the elevation of its top at an unknown distance from the base is $30^{\circ}$ and at a distance $10\;km$ further off from the mountain along the same line whose where angle of elevation is $15^{\circ}$ is _____ $(\tan 15^{\circ} = 0.27)$

Question

$\begin{array}{1 1}(A) \; 10\;km \\(B)\; 15\;cm \\(C)\;5\;km \\(D)\;20\;km \end{array}$