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': 'Gold standard study for Clinical research is: \nSelect and explain. \nA. Randomised double-blind trial, B. Systematic meta-analysis, C. Ecological study, D. Retrospective cohort study', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]