A number of economists have estimated the impact of unionization on workers' wages. Which of the following is one conclusion reached by these studies?

A) Holding constant the impact of other factors that affect wages, being in a union has no impact on a worker's wages.
B) Being in a union increases a worker's wages by about 10 percent, holding constant other factors that influence wages.
C) Union workers earn less than they would if they were not unionized. This is because of the impact of workers' strikes, during which union members do not receive wages.
D) The share of national income received by workers has increased significantly over time; unions have been responsible for about one-half of the increase in workers' share of national income from the end of World War II to 2000.

B