You would like to construct a 95% 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 633 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? X parts per million (b) For each of the following sampling scenarios, determine which distribution should be used to calculate the critical value for the 95% 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 or t X The sample has size 12, and it is from a normally distributed O O O O population with an unknown standard deviation. The sample has size 15, and it is from a population with a O O O O distribution about which we know very little. The sample has size 80, and it is from a non-normally distributed O O O O population with a known standard deviation of 49