GMAT Critical Reasoning (CR) | Re: A study of the difference in earnings between men and women

A study of the difference in earnings between men
and women in the country of Naota found that the
average annual earnings of women who are employed
full time is 80 percent of the average annual earnings
of men who are employed full time. However, other
research consistently shows that, in Naota, the
average annual earnings of all employed women is 65
percent of the average annual earnings of all
employed men.

Which one of the following, if also established by
research, most helps explain the