Economics Micoroeconomies question?

The marginal product of labor (MPL) will necessarily decline for the 12th worker because of the law of diminishing returns--all things constant the more we use of a certain input the less return we'll generate from it. However it's possible the MPL could decline by any value less than 13, not neccessarily 12. There is nothing in the question the suggest the MPL could decline to 10, or 12.5. So I'm tempted to answer D.