Why not B.

Hi, the crux of the issue is whether the signs are necessary. C gets to this exactly, if the signs were already going to be replaced then we might as well get better ones. B doesn't really matter. If we are incessantly replacing signs who really cares if they're the same price or more expensive than the old ones. Either way it's an unneeded expense, the only thing that matters is the magnitude, which is not really important to the authors argument.