A study was designed to estimate the true mean amount of radioactive element in lichen specimens. A sample of 9 lichen specimens were investigated, leading to the sample mean and standard deviation being 0.009 and 0.005 microcurie per milliliter, respectively. (a) Based on the sample information, construct a 99% confidence interval for the true mean amount of radioactive lelement. (b) What are underlying assumptions are required for the confidence interval in part(a) being valid