A Major Challenge That Is Involved in the Empirical Investigation

A major challenge that is involved in the empirical investigation of wage discrimination is:

A) The Oaxaca decomposition is subject to ability bias.
B) The unexplained wage differentials are larger than the explained differentials.
C) The absence of a comparison group leads to a systematic bias in the estimation of discrimination.
D) Some of the control variables are not really pre-market characteristics, as their values might reflect discrimination rather than cause it.
E) The unexplained wage differentials are too small to estimate.