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': 'When anchor unit is allowed to tip, it is called anchorage \nSelect and explain. \nA. Simple, B. Compound, C. Complex, D. Reciprocal', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]