There is a maximally strong argument for A, and not for B, because B is a source of infinite free energy just by using the rules of the portal.
Imagine the object is a super-heavy object, like a mountain. It's at rest relative to the most things. Relatively zero effort was spent to accellerate the zero-mass orange portal, wheras at the end, the mountain-sized object flying out of B could be harnessed to turn a crank, which lifts the portal back up for another whack at further accelleration.
https://www.youtube.com/watch?v=B19nlhbA7-E
Therefore, A can be the only answer that doesn't violate the conservation of energy, because B can be simply re-arranged into a source of infinite free energy, assuming both of the portal's apitures are 100% efficient and zero mass.
