A man crosses the river in shortest time at an angle $\theta=60 ^{\circ}$ to the direction of flow of water. If the speed of water is $v_w=4\; km/hr$ find the speed of man.

Question

$$(a)\;6km/hr \quad (b)\;10km/hr \quad (c)\;12km/hr \quad (d)\;8 km/hr$$