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
[{'content': 'The most common concept of hard and soft substance is \nSelect and explain. \nA. Relative resistance to indentation, B. Relative resistance to abrasion, C. Relative resistance to fracture, D. All of the above', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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.\xa0Hardness may be broadly defined as the resistance to permanent surface indentation or penetration.', 'role': 'assistant'}]