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
Randomized controlled trials are all except: 
Select and explain. 
A. Clinical trials., B. Preventive trials., C. Before and after comparison studies., D. Evaluation of health services.
C: Before and after comparison studies.
Answer: C: Before and after comparison studies. 
Explanation: None