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
Evidence based ortho research is based on the following studies: 
Select and explain. 
A. Cohort, B. Cross Sectional, C. Systematic studies, D. RCT
C: Systematic studies
Answer: C: Systematic studies 
Explanation: Evidence-based medicine pyramid. 
The levels of evidence are appropriately represented by a pyramid as each level, from bottom to top, reflects the quality of research designs (increasing) and quantity (decreasing) of each study design in the body of published literature. For example, systematic reviews are higher quality and more labor intensive to conduct, so there is a lower quantity published.