Question: A company is interested in estimating the mean number of days of sick leave taken by its employees. The firm's statistician randomly selects 200 personnel files and notes the number of sick days taken by each employee. The estimated sample mean is 12.2 days and the sample standard deviation is 10 days. How many personnel files would the director have to select in order to estimate μ to within 1 day with a 99% confidence interval?
A. 166
B. 200
C. 385
D. 550
E. 664