The speed of a boat is 5 km/hr is still water. It crosses a river of width 1 km along the shortest possible path in 15 minutes. The velocity of river water is :

Question

$\begin{array}{1 1} (1)\; \text{3 km/hr} \\ (2)\;\text{4 km/hr} \\ (3)\; \text{5 km/hr} \\ (4)\; \text{2 km/hr} \end{array}$