A randomly selected group of 40 adults participated in a sleep study. The mean number of hours slept was x=7.2 hours. The number of hours slept by adults seems to be normally distributed with

a) Mean: 7.2 hours, Standard Deviation: 1.5 hours
b) Mean: 7.2 hours, Standard Deviation: 0.5 hours
c) Mean: 7.2 hours, Standard Deviation: 2.0 hours
d) Mean: 7.2 hours, Standard Deviation: 0.2 hours