In determining the standard deviation - and, thus, by extension, the upper and lower control limits - for the average of a set of measurements used in a control chart, the determining factors are the standard deviation of the initial items being measured and __________________

a. Total number of measurements taken

b. Number of measurements in each set or subgroup (number of measurements per day, if a set of measurements is taken each day)

c. None of these

d. Number of sets or subgroups measured (number of days, if taken daily)

e. The difference between the largest measurement and the smallest measurement