This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Biostatistics, Subject name: Social & Preventive Medicine
The PEFR of a group of 11 year old girls follow a normal distribution with mean 300 L/min and stardard detion 20 L/min- 
Select and explain. 
A. About 95 % of the girls have PEFR between 260 and 340 L\/min, B. The girls have healthy lungs, C. About 5% of girls have PEFR below 260 L\/min, D. All the PEFR must be less than 340 L\/min
A: About 95 % of the girls have PEFR between 260 and 340 L\/min
Answer: A: About 95 % of the girls have PEFR between 260 and 340 L\/min 
Explanation: Ref:Parks 23rd edition pg 849. SD = 1 : confidence limit = 68% SD = 2 : confidence limit = 95% SD = 3 : confidence limit = 99.7% Here 2SD = 2x20 = +/-40 So, between 260 and 340 2SD covers 95% values, remaining is 5%, So 5% fall below 260 and above 340 Precisely 2.5% below 260 and 2.5% above 340