+511 In a random sample of 80 students in first grade, the mean hours of sleep per night was 9.4 with a standard deviation of 0.41. In a random sample of 50 students in 12th grade, the mean hours of sleep per night was 7.5 with a standard deviation of 0.52.
Which interval is a 99% confidence interval for the difference between the mean hours of sleep?
(1.75 h, 2.05 h)
(1.89 h, 1.91 h)
(1.43 h, 2.37 h)
(1.68 h, 2.12 h)
