A random sample of 21 items is drawn from a population whose standard deviation is unknown. The sample mean is a = 910 and the sample standard deviation is s= 20. Use Appendix D to find the values of Student's t. (a) Construct an interval estimate of / with 99% confidence. (Round your answers to 3 decimal places.) The 99% confidence interval is from to (b) Construct an interval estimate of / with 99% confidence, assuming that s = 40. (Round your answers to 3 decimal places.) The 99% confidence interval is from (c) Construct an interval estimate of / with 99% confidence, assuming that s = 80. (Round your answers to 3 decimal places.) The 99% confidence interval is from to (d) Describe how the confidence interval changes as s increases. O The interval stays the same as s increases. O The interval gets wider as s increases. ices The interval gets narrower as s increases