We use the t distribution to construct a confidence interval for the population mean when the underlying population standard deviation is not known. Under the assumption that the population is normally distributed, find ta/2, af for the following scenarios. (You may find it useful to reference the ttable. Round your answers to 3 decimal places.) tal2, df a. A 90% confidence level and a sample of 28 observations. b. A 95% confidence level and a sample of 28 observations. C. A 90% confidence level and a sample of 15 observations. d. A 95% confidence level and a sample of 15 observations