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 first reaction that takes place when an orthodontist attempts active tooth movement with a removable appliance is: \nSelect and explain. \nA. Pressure and tension zones form in the periodontal ligament, B. Force is applied on the tooth, C. Osteoclasts create undermining resorption, D. Tooth moves by direct resorption', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]