A cart of mass m moves on a horizontal track. A constant resistive force Fr acts opposite to its motion, reducing its speed from vi to vf(vf<vi). Which statement is correct about the work done by the resistive force (Wr) and the change in kinetic energy (ΔK)?
a) Wr is negative and equals Ki−Kf
b) Wr is negative and equals −21m(vi−vf)2
c) Wr is negative and equals Kf−Ki
d) Wr is positive and equals Ki−Kf