Roman grain moved thousands of kilometres by sea and a fraction of that distance by land cost more. What single ratio explains the pattern?

C. The cost of water transport against land, an order apart

An ox eats its cargo over a few hundred kilometres and a sail costs nothing, so the economic map was drawn by water rather than by distance. It is why Rome fed itself from Egypt rather than from inland Italy, and why the railway's real achievement was making land transport comparable at all.