A system consisting of two particles, m1 and m2, with momenta at time t1, p1 and p2, respectively. At time t2, m1 has momentum p1 + Δp, and m2 has momentum p2 - Δp. Which of the following statements will always be true for this scenario?
A. The system has not interacted with any object in its surroundings.
B. Only internal forces have affected the momentum of m1 and m2.
C. The object does not obey the laws of physics.
D. The momentum of the center of mass of the system is unchanged from t1 to t2.
E. One cannot draw any conclusion from the given information.
It says that the answer for this question is D.
I know that A and B are not correct, but I got totally lost on C and D.
I don't understand the point that this scenario does obey the laws of physics and
why the momentum of the center of mass of the system is unchanged from t1 to t2.