PT 22 S2 Q21

A study of the difference in earnings between men and women in the country ... ...

Which one of the following, if also established by research, most helps explain the apparent discrepancy between the research results described above?
(A) In Naota, the difference between the average annual earnings of all female workers and the average annual earnings of all male workers has been gradually increasing over the past 30 years.
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in Practice Set: "Resolve/Explain" Questions, Set 2 - Difficult/Uncommon Mix (25Q)