A random sample of 21 items is drawn from a population whose standard deviation is unknown. The sample mean is xx = 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.)

(b) Construct an interval estimate of with 99% confidence, assuming that s = 40. (Round your answers to 3 decimal places.)

(c) Construct an interval estimate of with 99% confidence, assuming that s = 80. (Round your answers to 3 decimal places.)

(d) Describe how the confidence interval changes as s increases. multiple choice

• The interval stays the same as s increases.
• The interval gets wider as s increases.
• The interval gets narrower as s increases.