In a random sample of 12 smokers, bone mineral density was found to be .65 with a standard deviation of .15. In a random sample of 15 nonsmokers, bone mineral density was found to be .5 with a standard deviation of .13. Using these data, you need to construct a 95% confidence interval estimate of thedifference between the mean serum cotinine level of exposed nonsmokers and unexposed nonsmokers. Assuming normality of the distribution of bone mineral density and equal variances, what value should you use for the margin oferror?

a..2855

b..2828

c..2852

d..2973

e..30