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': 'Which of the following used as a reference for a study through a period of time \nSelect and explain. \nA. Base line data, B. Prevalence, C. Incidence, D. Rate', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]