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': 'What is the outcome when optimum orthodontic force is equal to capillary pressure? \nSelect and explain. \nA. No significant movement, B. Undermining resorption, C. Directly acts on osteoblasts to cause movement, D. Directly acts on osteoclasts to cause movement', 'role': 'user'}, {'content': 'Answer: D \nExplanation: The optimum force levels for orthodontic tooth movement should be just high enough to stimulate cellular activity without completely occluding blood vessels in the PDL.\nBoth the amount of force delivered to a tooth and the area of the PDL over which that force is distributed are important in determining the biologic effect.\nThe PDL response is determined not by force alone, but by force per unit area, or pressure.\nBecause the distribution of force within the PDL, and therefore the pressure, differs with different types of tooth movement, it is necessary to specify the type of tooth movement as well as the amount of force in discussing optimum force levels for orthodontic purposes.', 'role': 'assistant'}]