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
Stress is defined as: 
Select and explain. 
A. An applied load or force, B. A deformation resulting from an applied load, C. An external force opposing an applied load, D. An internal force opposing an applied load
D: An internal force opposing an applied load
Answer: D: An internal force opposing an applied load 
Explanation: Stress—Force per unit area within a structure subjected to a force or pressure.
Pressure—Force per unit area acting on the surface of a material.
Phillips dental materials 12th edition page no 48