A process that is under statistical control has an Average of Averages = 12.30, an Average of Ranges = 1.56, and sample size n = 7. The three sigma limits of the means chart that correspond to the X-R control chart are:

a. 11.40 and 13.20

b. 11.65 and 12.95

c. 10.45 and 12.65

d. 11.52 and 13.08