For a $3-$sigma $x-$bar chart where the process standard deviation is known, the upper control limit:

Select one:

a$.$ is $3$ below the mean of sample means for a $3$ control chart.

b$.$ is $3\frac{}{\sqrt[\phantom{2}]{m}}$ above the mean of sample means for a $3$ control chart.

c$.$ cannot be calculated unless the average range is known.

d$.$ is $3\frac{}{\sqrt[\phantom{2}]{m}}$ below the mean of sample means for a $3$ control chart.

e$.$ is $3$ above the mean of sample means for a $3$ control chart.