From a population that is not normally distributed and whose standard deviation is not known, a sample of 48 items is selected to develop an interval estimate for μ. Which of the following statements is true? a. The standard normal distribution can be used. b. The t distribution with 48 degrees of freedom must be used. c. The sample size must be increased in order to develop an interval estimate. d. The t distribution with 47 degrees of freedom must be used.