What is the reasoning behind setting the upper and lower control limits $+-3{}_{\stackrel{}{x}}$ from the mean?

A$.$ The limits of the $Z$ table are set at $+-3{}_{\stackrel{}{x}}$

B$.$ We know that $9973\%$ of natural variation is contained within $+-3{}_{\stackrel{}{x}},$ and thus any data point outside of the limits is likely due to assignable variation

C$.$ We know that $99\mathrm{.}73\%$ of assignable variation is contained within $+-3{}_{\stackrel{}{x}},$ and thus any data point outside of the limits is likely due to natural variation

D$.95\mathrm{.}45\%$ of the time, the sample averages will fall within $+-3{}_{-}$if the process has only natural variation