13. From a population that is not normally distributed and whose standard deviation is unknown, a sample of 15 items is selected to develop an interval estimate for the population mean μ. Which of the following is true?
(A)The sample size must be increased;
(B)The normal distribution may be used;
(C) Thet distribution with 14 degrees of freedom can be used;
(D) The t distribution with 15 degrees of freedom can be used.