(mc)which factor produced gains in union membership in the united states during the late 19th century and early 20th century?

The factor of shifting from an agricultural economy to an industrial economy.

Explanation:

After the advent of industrialization, most people preferred moving to industrial towns leaving agriculture behind. As more people opted to work in industries, the demand for labor automatically subsided and the industry owners started exploiting their workers. To protect their own interests, the workers found it necessary to form unions. The membership of such unions gained for workers protection against exploitation by the industrialists.