Two objects interact with each other and with no other objects. Initially, object A has a speed of $6\frac{m}{s},$ and object $B$ has a speed of $5\frac{m}{s}.$ In the course of their motion, they return to their initial positions. Then A has a speed of $4\frac{m}{s},$ and $B$ has a speed of $3\frac{m}{s}.$ We can conclude that

a$.$ mechanical energy was decreased by nonconservative forces.

b$.$ mechanical energy was decreased by conservative forces.

c$.$ the potential energy changed from the beginning to the end of the trip.

d$.$ mechanical energy was increased by nonconservative forces.

e$.$ mechanical energy was increased by conservative forces.