You Are Evaluating Investing in a Cognitive Training Company

You are evaluating investing in a cognitive training company.For this reason,you want to determine whether users who complete at least 75% of the recommended daily training for two months show improved levels of reading comprehension and problem solving skills.You select a random sample of new users and get test scores for each participant in the sample.The test score is a composite of reading comprehension and problem solving skills.Two months later,you randomly select 15 users who have completed 75% or more of the recommended training for the last two months and have them take a test similar to the initial test.You are interested in determining whether the average test score before training is different than the average test score after training for this sample.The after-training average is 92.8,which is 2.7 points higher than the before-training average.The sample standard deviations of the differences is 1.2.You can assume that the differences are normally distributed in the population.The degrees of freedom for this test are ______.

A) 30
B) 29
C) 28
D) 15
E) 14