acid rain caused by the reaction of certain air pollutants with rainwater, appears to be a growing problem in the northeastern united states. (Acid rain affects the soil and causes corrosion on exposed metal surfaces.) Pure rain falling through clean air registers a pH value of 5.7 (pH is a measure of acidity : 0 is acid; 14 is alkaline). Suppose water samples from 40 rainfalls are analyzed for pH, and x and s are equal to 3.7 and 0.5, respectively, find a 99% confidence interval for the mean pH in rainfall and interpret the interval.