Which of the following is true about labor union effects on labor markets?
A: The higher wages in the union sector tend to elevate wages indirectly across all sectors of the labor market
B: Union labor tends to be less productive because the union protects its workers from discipline or firing
C: Approximately 90% of the wage difference between union and nonunion workers is due to the higher quality of union workers
D: Labor unions have no documented impacts on labor markets because they are subject to the same labor market forces that other firms face