18. Suppose we know that, historically, the standard deviation of annual rainfall in a particular state is 2 inches. a. We measure rainfall in that area for 35 years, and we find an average of 37 inches per year. What is the 95% confidence interval for the average rainfall? [4 points] b. How many years of data would we need if we want to estimate the actual average within less than plus-or-minus 0.5 inches at 95% confidence? [3 points]