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
You have been asked to design a study for a disease whose prevalence in the community is 10%. The alpha error has to be kept at 5% with a relative precision of 20% and a power of 20%. What will be the accurate sample size for this study? 
Select and explain. 
A. 400, B. 900, C. 1800, D. 3600
B: 900
Answer: B: 900 
Explanation: Ans: B. 900