Union Workers Receive More Compensation Than Otherwise Similar Nonunion Workers

Union workers receive more compensation than otherwise similar nonunion workers.This fact:

A) is proof that unions raise the wages of their members relative to nonunion workers
B) could be explained by the fact that women workers are a larger proportion of weakly unionized industries
C) could be explained by the fact that unionized industries are less capital intensive
D) could be explained by the fact that unionized firms are typically smaller