On a quality control chart, when would a statistical out-of-control situation requiring corrective action be suspected?

A. Six successive plots fall above and below the mean within ± 1 s.
B. Six successive plots fall above and below the mean within ± 2 s.
C. One plot falls within the area of ± 2 s to 3 s within a 20-consecutive-day span.
D. One plot falls outside the area of ±3 s within a 20-consecutive-day span.