How did increases in employment levels for women, children, and immigrants contribute to the growth of labor unions in the late 19th century? a. these groups worked for less than native-born men, keeping wages low and leading many men to join unions to raise them. b. these groups were considered weak and inferior by many male factory owners, allowing them to unionize without appearing threatening. c. these groups generally worked in industries without large trusts, weakening management and allowing them to organize more easily. d. these groups were considered especially vulnerable by many americans, allowing them to become the public faces of early unions.