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': 'Retention appliance should be \nSelect and explain. \nA. Retentive only, B. Passive only, C. Passive + retentive, D. Active + retentive', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]