30. Acid Rain Acid rain—rain with a pH value less than 5.7, caused by the reaction of certain air pollutants with rainwater—is a growing problem in the United States. Suppose water samples from 40 rainfalls are analyzed for pH, and x and s are equal to 3.7 and .5, respectively. Find a 99% confidence interval for the mean pH in rainfall and interpret the interval. What assumption must be made for the confidence interval to be valid?