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: Social & Preventive Medicine
In a population of 5000, incidence of disease is 100  in 1 year duration of disease studied for is 2 years calculate Prevalence - 
Select and explain. 
A. 20/1000, B. 40/1000, C. 80/1000, D. 400/1000
B: 40/1000
Answer: B: 40/1000 
Explanation: Incidence = No. of disease person / Population at risk x 1000 = 100/5000 = 1000 = 20 per one thousand per year 
Prevalence = I x D = 20 x 2 = 40 per one second