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 optimal orthodontic force per square centimeter of the root surface area is held to be: \nSelect and explain. \nA. 18-22 gms, B. 18-26 gms, C. 20-26 gms, D. 26-30 gms', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]