Research on Reasons for the Gender and Ethnic Gaps in Earnings

Research on reasons for the gender and ethnic gaps in earnings has concluded that

A) gender differences in training and investment of time on the job fully explain the gender wage gap.
B) a major factor is the difference in jobs held by females and males and by people in different ethnic groups.
C) since the 1963 passage of the Federal Equal Pay Act, discrimination has not been a factor that contributes to these wage gaps.
D) none of the above