4. The breaking strengths of a random sample of 20 bundles of wool fibers have a sample mean 436.5 and a sample standard deviation 11.9. (a) Construct 90%, 95%, and 99% confidence intervals for the average breaking strength of the wool fibers. (b) Compare the widths of the three confidence intervals. At which level of confidence do you have the widest interval? (c) Do you think it is plausible that the average breaking strength is equal to 450?