In the Context of Estimating the Union Wage Effect, Which

In the context of estimating the union wage effect, which of the following statements concerning "selection bias" is false?

A) Whenever there is a queue of workers applying for union jobs, employers tend to hire the most productive of them.
B) Unionized workers might not have identical characteristics to non-unionized workers, insofar as their levels of marginal productivity are concerned.
C) Workers are hired into union jobs according to a randomized selection process.
D) Factors that are not observed by the researcher influence the outcome of whether or not the worker is represented by a union.
E) Workers are hired into union jobs according to a fixed selection process.