Which of the following is not true with respect to tests for the difference between two means when the population standard deviations are known? O a. The samples must be randomly and independently selected. O b. The margin of error for a confidence interval estimate is 2 N O 02 Zal2 + n n2 c. The test statistic for a hypothesis test has a t distribution. O d. The samples must come X from normally distributed populations or the sample sizes must be large enough to apply the central limit theorem