Suppose that workers in Group A earn more, on average, than workers in Group B$.$ Which of the following could NOT explain why Group A earn more than Group B $($assuming that each statements below is true$).$

Question $4$ options:

Workers in Group A$,$ on average, have more labor market experience than workers in Group B

Workers in Group A are more likely to work in jobs with amenities than workers in Group B

Workers in Group A are more likely to be union members than workers in Group B

Workers in Group A have received more on$-$the$-$job training than workers in Group B

There is discrimination against workers in Group B$,$ but not workers in Group A