【单选题】

 assignable cause or special variation is:

a. is indicated when a point falls outside the control limits.
b. is indicated when a point falls outside the specification limits.
c. is indicated when a run of seven or more points fall above the center line of a standard control chart.
d. is indicated when a run of seven or more points fall below the center line of a standard control chart.
e. a, c, and d.