Q. Some industries use "six sigma" control to establishing upper and lower control limits (UCL and LCL) to provide very tight process control. ("Sigma" is the standard deviation of the probability distribution.) The more traditional limits for process control charts are as follows:

a. Two sigma between UCL and LCL 

b. Six sigma between UCL and LCL

c. Three sigma between UCL and LCL

d. Three sigma between specification limits (UCL and LCL)