Two Particles Interact by Conservative Forces

Two particles interact by conservative forces. This addition, an external force acts on each particle.. They complete round trips, ending at the points where they started. Which of the following must have the same values at the beginning and end of this trip?

A) a total kinetic energy of the two-particle system
B) the potential energy of the two-particle system
C) the mechanical energy of the two-particle system
D) the total linear momentum of the two-particle system
E) none of the above