a manufacturer of cell phones would like to estimate how much longer the battery lasts in their model 10 phone than in their model 9 phone. to estimate this difference, they randomly select 40 cell phones of each model from the production line. they subject each phone to a standard battery life test. the 40 model 10 phones have a mean battery life of 14.4 hours with a standard deviation of 2.1 hours. the 40 model 9 phones have a mean battery life of 12.8 hours with a standard deviation of 2.3 hours. the conditions for inference have been met. what is the correct 95% confidence interval for the difference in the population means?