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
Which type of RPD definitely requires an indirect retainer? 
Select and explain. 
A. 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
A: Class I,II R.P.D
Answer: A: Class I,II R.P.D 
Explanation: None