In a random sample of 60 students in first grade, the mean hours of sleep per night was 9.6 with a
standard deviation of 0.51. In a random sample of 40 students in 12th grade, the mean hours of sleep per
night was 7.2 with a standard deviation of 0.62.
Which interval is a 99% confidence interval for the difference between the mean hours of sleep?
O (2.36 h, 2. 44 h)
O (2. 19 h, 2.61 h)
O (2. 10 h, 2. 70 h)
O (2.29 h, 2.51 h)