At one college, GPA's have a distribution that is unimodal and symmetric with a mean of 2.9 and a standard deviation of 0.6. What is the range of GPA's from one standard deviation below the mean to one standard deviation above the mean?

a) 2.3 to 2.9
b) 2.9 to 3.5
c) 0.6 to 3.5
d) 2.3 to 3.5