A cart starting from rest rolls down a frictionless 1.0 m high hill. It travels a distance 2.0 m along the rough bottom surface until it comes to rest. What is the coefficient of kinetic friction along the bottom surface?
a. 4 m/s.
b. 0.5 m/s.
c. 25 m/s.
d. 7 m/s.
e. 6 m/s.