After thinking about it longer then i care to admit I think i finally agreed with you.

As you said it is all relative, from the prospective of the moving portal. You could say it isnt moving at all but the entire world around it is moving, thus when people enter the portal from the portal's prospective they people are the ones moving and will continue moving when they exist.

Hmm tlnit that i typed this out I feel like i didn't do a very good job. Owell the answer is B.