A horizontal bridge is built across a river. A student standing on the bridge throws a small ball vertically upwards with a velocity 4 m s^-1. The…

A horizontal bridge is built across a river. A student standing on the bridge throws a small ball vertically upwards with a velocity $\displaystyle 4 \mathrm{~m} \mathrm{~s}^{-1}$. The ball strikes the water surface after $\displaystyle 4$ s . The height of bridge above water surface is (Take $\displaystyle \mathrm{g}=10 \mathrm{~m} \mathrm{~s}^{-2}$ ) :