Two trains each 50 m long are travelling in opposite direction with velocity 10 m/s and 15 m/s. The time of crossing is

Question

$\begin{array}{1 1} (a) \;2\;s \\ (b) \;4\;s \\ (c) \;2 \sqrt 3\;s \\ (d) \;4 \sqrt{3} \;s\end{array}$