Two objects interact with each other and with no other objects. Initially object A has a speed of 5 m/s and object B has a speed of 10 m/s. In the course of their motion they return to their initial positions. Then A has a speed of 4 m/s and B has a speed of 7 m/s. We can conclude:.
A. mechanical energy was decreased by nonconservative forces.
B. mechanical energy was increased by nonconservative forces.
C. the potential energy is unchanged from the beginning to the end of the trip.
D. mechanical energy was decreased by conservative forces.
E. mechanical energy was increased by conservative forces.