Which of the Following Is an Example of Meta-Analysis

Which of the following is an example of meta-analysis?

A) Yeeun used one study, which used a consistent measuring method, to determine the cause of employee stress.
B) Sosuke used one study, which combined different samples, to determine why customers buy a certain product.
C) Gail used two studies, each using the same samples and measures, to determine why sales of a product have declined.
D) Sergio combined several studies, each using the same measuring method, to determine why profitability has increased.
E) Fionn combined several studies, each using different kinds of samples and measures, to determine the cause of employee turnover.