The prevalence of autism has been growing at an alarming rate over the past couple of decades, according to the U.S. Center for Disease Control. Researchers still cannot entirely explain the reason for the increased prevalence. The age of the mother has been identified as a contributing risk factor for autism. You will use a confidence interval to estimate the difference between the mean age (at time of birth) of mothers of autistic children and the mean age of mothers of non-autistic children. Which confidence interval should you use?