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': 'Which type of RPD definitely requires an indirect retainer? \nSelect and explain. \nA. Class I,II R.P.D, B. Class II, IV R.P.D, C. Class I & IV R.P.D, D. Class III & IV R.P.D', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]