Which One of the Following Statements Is True

Which one of the following statements is true?

A) Common cause variation arises from external sources that are not inherent in the process and is where statistical quality control methods are most useful.
B) If the metric is discrete, a p- or c-chart is normally used.
C) Statistical process control is not used in hospitals due to the high liability costs.
D) Statistical process control charts are used when trying to monitor and control 5- and 6-sigma quality levels.