A boat travels 24 km upstream in 6 hours and 20 km downstream in 4 hours. Then the speed of boat in still water and the speed of water current are respectively
[A] 5 km/hr and 2 km/hr
[B] 4 km/hr and 2 km/hr
[C] 4 km/hr and 3 km/hr
[D] 4.5 km/hr and 0.5 km/hr