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
Which of the following used as a reference for a study through a period of time 
Select and explain. 
A. Base line data, B. Prevalence, C. Incidence, D. Rate
B: Prevalence
Answer: B: Prevalence 
Explanation: None