What Defines the Total Momentum in the Center of Mass Frame?

[Thefox14]

Homework Statement

A key concept about the center-of-mass frame is that

A. the total momentum in the center of mass frame is always zero.
B. both the initial and final momentum vectors form pairs of equal magnitude and opposite direction
C. this frame always moves at the same velocity as the initial index particle
D. (a) and (b)
E. (a) and (c)
F. all of the above

2. The attempt at a solution

Currently I think it's A, as that's one of the key things about this frame, but what makes me uncertain is I'm not sure what an initial index particle is. I'm also pretty sure it's not C as that would only be true for elastic collisions correct?

 

[Thefox14]

Would someone be able to explain what an index particle is? Just knowing that would help a lot