Two objects interact with each other and with no other objects. Initially object A has a speed of $9$ m$/$s and object B has a speed of $8$ 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 $3$ m$/$s$.$ We can conclude

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mechanical energy was decreased by conservative forces. $$

mechanical energy was decreased by nonconservative forces.$$

mechanical energy was increased by conservative forces.

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

mechanical energy was increased by nonconservative forces. $$