A 210 m long train is moving north at a speed of $25\;m/s$. A small bird is flying south a little above the train with speed of $5m/s$. The time after which the bird crosses the train is :

Question

$\begin{array}{1 1} (a) \;6\;sec \\ (b) \;8\;sec\\ (c) \;7\;sec \\ (d) \;12\;sec\end{array} $