For a 3-sigma x-bar chart where the process standard deviation is known, the upper control limit: a.is 3σ below the mean of sample means for a 3σ control chart. b.is 3σ above the mean of sample means for a 3σ control chart. c.is 3σ√π below the mean of sample means for a 3σ control chart. d.is 3σ/π above the mean of sample means for a 3σ control chart. e.cannot be calculated unless the average range is known.