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: Social & Preventive Medicine
Randomized controlled trials are: 
Select and explain. 
A. Preventive trials, B. Before and after comparison studies, C. Evaluation of Health Services, D. Clinical trials
D: Clinical trials
Answer: D: Clinical trials 
Explanation: Clinical trials