A car , starting from rest, accelerates at the rate `f` through a distance `S` then continues at constant speed for time `t` and then decelerates at the rate `(f)/(2)` to come to rest . If the total distance traversed is `15 S` , then
A. `S=ft`
B. `S=(1)/(6)ft^(2)`
C. `S=(1)/(72)ft^(2)`
D. `S=(1)/(4)ft^(2)`