24.From the National Health and Nutrition Examination Survey, a sample of 50 nonsmokers exposed to environmental tobacco smoke (ETS) have a mean serum cotinine level of 4.10 ng/ml with standard deviation 10.21 ng/ml, and a sample of 50 nonsmokers not exposed to ETS have a mean serum cotinine level of 0.41 ng/ml with standard deviation 1.21 ng/ml. Using these data, you need to construct a 95% confidence interval estimate of the difference between the mean serum cotinine level of exposed nonsmokers and unexposed nonsmokers. Assuming that the population variances are known to be equal to the two sample variances, what value should you use for the margin of error?

a.0.17

b.1.45

c.1.96

d.2.11

e.2.85