ANS:C - 0.002

In Rockwell hardness testing, the depth of indentation is used as a measure of the material's hardness. The Rockwell hardness number (RHN) is determined based on the depth of penetration of an indenter into the material under a specific load. The difference in one unit of Rockwell hardness number (RHN) corresponds to a difference in the depth of indentation of approximately 0.002 mm. This means that for every unit increase or decrease in Rockwell hardness number, the depth of the indentation made by the indenter into the material changes by approximately 0.002 mm. This precision in measurement allows for accurate and consistent evaluation of the material's hardness, with each unit change indicating a small but significant difference in the material's resistance to indentation.