A steamer is moving due east with $36 \mathrm{~km} / \mathrm{h}$. To a man in the steamer the wind appears to blow at $18 \mathrm{~km} / \mathrm{h}$ due north. Find the velocity of the wind.
(A) $5 \sqrt{5} \mathrm{~ms}^{-1} \tan ^{-1} \frac{1}{2}$ North of East
(B) $5 \mathrm{~ms}^{-1} \tan ^{-1} 2$ North of East
(C) $5 \sqrt{5} \mathrm{~ms}^{-1} \tan ^{-1} 2$ North of East
(D) $5 \mathrm{~ms}^{-1} \tan ^{-1} \frac{1}{2}$ North of East