Suppose That at Each of the Time Points 1,2,3,… $\mu$ And Standard Deviation

Suppose that at each of the time points 1,2,3,….., a sample of size n is selected at random from a normal distribution with known mean $\mu$ and standard deviation $\sigma$ In order to construct a 3-sigma $\bar { X }$ chart, we need to

A) determine $\bar { x } _ { 1 } , \bar { x } _ { 2 } , \bar { x } _ { 3 } , \ldots \ldots$
Calculated values of the corresponding sample means.
B) plot $\bar { x } _ { 2 } ^ { \prime } s$
Over time; that is, plot the points $\left( 1 , \bar { x } _ { 1 } \right) , \left( 1 , \bar { x } _ { 2 } \right) , \left( 1 , \bar { x } _ { 3 } \right) , \ldots \ldots$
And so on.
C) Draw horizontal lines across the plot as $\mu \pm 3\sigma / \sqrt { n }$
)
D) All of the above are needed.
E) Only A and B are needed.