Answer: unskilled immigrants willing to work for low wages
Explanation:
Assembly line production became very popular in the United States in the first half of the 20th century when Ford used it in the production of his cars. The speed and efficiency of the assembly line made him able to sell his cars at a cheaper rate.
Assembly line production involves one person at a station doing one particular job repetitively. This was good news for unskilled immigrants who simply had to do rudimentary tasks at their stations and get paid for it without the need for skills as they could simply learn by routine.