Figure 2.16 shows a chart of the Dow Jones Industrial Average from 1990 through 2011. This plot reveals some obvious change points, such as the 2008 financial collapse, and some unexplainable short-term variation. Assuming that these are the only sources of variation in this plot, what source of variation is NOT visible in the plot?Special causeCommon causeStructural variationAll of the aboveNone of the above