A bullock cart has to cover a distance of 120 km. in 15 hours. If it covers half of the journey in $\frac{3}{5}th$ time, the speed to cover the remaining distance in the time left has to be :
[A] 6.4 km/hr
[B] 6.67 km/hr
[C] 10 km/hr
[D] 15 km/hr