A company believes its customer service response time has improved after a new training program. Before the program, the average response time was 15 minutes. A random sample of 40 customer service interactions after training shows an average response time of 13.5 minutes with a standard deviation of 3 minutes. a) Formulate the null and alternative hypotheses to test whether the training significantly reduced response time. b) Perform a t-test at the 5% significance level and interpret the result. c) What assumptions are necessary for the t-test to be valid in this case.