You would like to construct a 99% confidence interval to estimate the population mean PCB (Polychlorinated biphenyl) level in white croaker fish from the San Francisco Bay. The random sample we select has a mean of 640 parts per million and a standard deviation of 55 parts per million. (a) What is the best point estimate, based on the sample, to use for the population mean? U parts per million (b) For each of the following sampling scenarios, determine which distribution should be used to calculate the critical value for the 99% confidence interval for the population mean. (In the table, Z refers to a standard normal distribution, and t refers to a t distribution.) Could use Sampling scenario Z t Unclear either Z ort The sample has size 100, and it is from a non-normally distributed population with a known standard deviation of 49. The sample has size 12, and it is from a population with a distribution about which we know very little. The sample has size 18, and it is from a normally distributed population with an unknown standard deviation.