A university professor is conducting a study to determine the average hours of sleep per night for his undergraduate students. He randomly selects a sample of 20 students and finds that the average sleep time is 13.1 hours with a standard deviation of 3.7. What is the 90% confidence interval for the sample mean? O a. (10.97), (15.23) O b. (11.74, 14.46) O c. (11.48, 14.72) O d. (12.04, 14.16)