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
3 Standard deviation from mean covers how much areas under normal distribution curve- 
Select and explain. 
A. 63-6%, B. 66-6%, C. 95%, D. 99%
D: 99%
Answer: D: 99% 
Explanation: Ans. is 'd' i.e., 99% Area under standard normal distribution curve* It can be understand by the 68-95-99.7 rule -i) The area between one standard deviation on either side of the mean ( x +- 1 o) will include 68 percent of the values.ii) The area between two standard deviations on either side of the mean will cover 95% of values.iii) The area between three standard deviation will include 99.7% of values.* These limits on either side of the mean is called "confidence limits".* For example, if we are considering the 95 percent confidence limits (Area under 2 SD), that means that 95% of the values in the distribution will be included between the limits (2 SD x +- 2 a). Therefore, the probability of a reading falling outside confidence limit (95%) is only 5%, i.e. 1 in 20 (P = 0.05).* So,For 1 SD, confidence limit - 68%For 2 SD, confidence limit - 95%For 3 SD, confidence limit - 99.7%