This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
The most common concept of hard and soft substance is 
Select and explain. 
A. Relative resistance to indentation, B. Relative resistance to abrasion, C. Relative resistance to fracture, D. All of the above
A: Relative resistance to indentation
Answer: A: Relative resistance to indentation 
Explanation: The most common concept of hard and soft substances is their relative resistance to indentation. Hardness is therefore a measure of the resistance to plastic deformation and is measured as a force per unit area of indentation. Hardness may be broadly defined as the resistance to permanent surface indentation or penetration.