From the meaning of the question, no matter where a piece is randomly selected, its gravity is equal, because the pressure of an object resting on a horizontal plane is equal to its own gravity, so its pressure on the ground is also equal;

When the pressure is greatest, take one piece from the bottom layer, and when the pressure is least, take any piece from the top two layers.

The length, width and height of a cuboid are 3: 1: 1 respectively.

∴ The ratio of the maximum stress zone to the minimum stress zone is 3: 2,

From p=FS, the ratio of maximum and minimum pressure is 3: 2.

So choose C.