A Quality Analyst wants to construct a control chart for determining whether three machines, all producing the same product, are under control with regard to a particular quality variable. Accordingly, he sampled four units of output from each machine, with the following results : Machine \#1 measurements [12, 20, 21, 16]; Machine \#2 measurements [ 12, 20, 20 , 13]: Machine \#3 measurements [24,19,23,14]. Using the factors for three sigma control limits, what are x-bar chart upper and lower control limits? a. 24.4 and 11.26 b. 44.83 and -9.167 c. 20.26 and 15.4 d. 27.01 and 8.653