Which of the following is true regarding the process capability index C_{pk}? 

a. The C_{pk} index can only be used when the process centerline is also the specification centerline. 

b. The larger the C_{pk} , the more units meet specifications. 

c. C_{pk} index value of 1 is ideal, meaning all units meet specifications. 

d. Positive values of the C_{pk} index are good; negative values are bad.