Quality control charts usually have a central line and upper and lower control limit lines. Which of the following is not a reason that the process being monitored with the chart should be investigated?

	a.	A sudden and sustained shift in the plots toward the upper or lower control limit
	b.	There is an upward trend in the plots
	c.	A large number of plots are on or near the central line
	d.	A single plot falls outside of the control limits