Which of these is a correct expression for the UCL of the modified control charts when the type I error is not specified?

• A. (UCL=USL+ left(Z_ + frac{3}{ sqrt{n}} right) sigma )
• B. (UCL=USL+ left(Z_ - frac{3}{ sqrt{n}} right) sigma )
• C. (UCL=USL- left(Z_ - frac{3}{ sqrt{n}} right) sigma )
• D. (UCL=USL- left(Z_ + frac{3}{ sqrt{n}} right) sigma )