A marketing research company is estimating the average total compensation of CEOs in the service industry. Data were randomly collected from 18 CEOs and the 95% confidence interval for the mean was calculated to be ($2,181,260, $5,836,180). What additional assumption is necessary for this confidence interval to be valid? The population of total compensations of CEOs in the service industry is normally distributed because of the small sample size. The distribution of the sample means is approximately normal. None. The Central Limit Theorem applies. The sample standard deviation is less than the degrees of freedom.