Why is minimizing the within subgroup sample variation so important to sampling to set control limits?

Group of answer choices

(a) Because any variation within the samples will minimize the average, or centerline, on the x-bar chart

(b) Because variation is always to be avoided, as it contaminates the calculations

(c) Because within subgroup variation must always be a small fraction of the between subgroup variation

(d) Because the within subgroup sample variation is used to set the control limits for both the Xbar and the range chart which should only be based on random variation