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
While calculating the incubation period for measles in a group of 25 children, detion is 2 and the mean incubation period is 8 days. Calculate the standard error - 
Select and explain. 
A. 0.4, B. 1, C. 0.5, D. 2
A: 0.4
Answer: A: 0.4 
Explanation: Ans. is 'a' i.e., 0.4