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': 'To study the incidence of caries in 10 yr old in a school which is best study design. \nSelect and explain. \nA. Porspective, B. Retrospective, C. Cross sectional study, D. Cross over study', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]