In establishing control charts for a process, it is often essential to use both the X$-$bar chart and the R chart. Which of the following statements explain what these charts show?

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$$ The x$-$bar$\_$ chart tells us the behavior of sample mean and the R chart tells us whether or not the x$-$bar chart is within control.

$$ The R chart tells us the behavior of sample variability and the x bar chart tells us whether or not the R chart is within control.

$$ The x bar chart tells us the behavior of sample mean and the R chart tells us the behavior of sample variability.

$$ The R chart is an alternative approach to measure the number of defective units in a process.

$$ None of the above statements is an acceptable explanation.