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
Gold standard study for Clinical research is: 
Select and explain. 
A. Randomised double-blind trial, B. Systematic meta-analysis, C. Ecological study, D. Retrospective cohort study
B: Systematic meta-analysis
Answer: B: Systematic meta-analysis 
Explanation: None