Why do most measurement control situations include a R-Chart along with the X-Bar Chart?

It is important to measure and understand variability within a sample along with the sample averages.

The R-Chart is another way of measuring X-Bar, so this turns out to be an extra check on the control of the sample averages.

The R-chart is easier to calculate, so it is a way to be sure the worker doesn't make a mistake.

The ups and downs of the X-Bar Chart should exactly match the ups and downs of the R-Chart; if not, the process is out of control.

None of the above is a correct answer.