Among the known ores, diamond is the hardest, followed by corundum. A kind of ore can only be marked by ore that is as hard or harder as it is. The existing ore can be marked by corundum. So which of the following is wrong in the following conclusion? ['This ore is not diamond.', 'This ore may be corundum.', 'This ore is diamond or corundum.', 'This ore may not be as hard as corundum.']