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
The prevalence of dental caries in a community in the year 2000 and 2003 was 18% and 38% respectively. The incidence of caries for the same population in three years would be: 
Select and explain. 
A. 38%, B. 20%, C. 10%, D. 56%
B: 20%
Answer: B: 20% 
Explanation: Hint:
Incidence  = No. of new cases in a specified time (38-18 = 20%)
Prevalence = Incidence x Duration.  
Incidence = Prevalence / Duration = P/D.