Which of the Following Statements Is/are an Advantage of Systematic

Which of the following statements is/are an advantage of systematic reviews?

A) They can shed light on the external validity of the findings of an individual study.
B) They can help make sense out of a bewildering array of findings from different types of studies that produced inconsistent results.
C) They can apply statistical techniques that merge the findings of many different studies.
D) All of the above.