Selection bias is a problem when trying to estimate the return to education in a standard human capital model. In this context, what does selection bias refer to? A. Workers self-select education levels and jobs based on their abilities and preferences. B. Colleges select who they are willing to accept. C. The wage-schooling locus does not have a constant slope. D. The data sample is nonrandom. E. The wage-schooling locus is estimated to have a negative slope