For a $3-$sigma x$-$bar chart where the process standard deviation is known, the upper control limit:Select one: a$.$ is $30/$ In above the mean of sample means for a $3$o control chart. b$.$ is $30$ above the mean of sample means for a $3$o control chart. c$.$ is $30$ below the mean of sample means for a $3$o control chart. d$.$ cannot be calculated unless the average range is known. e$.$ is $30/$ m below the mean of sample means for a $30$ control chart.