What is the MOST LIKELY explanation for the rapid population growth in Pittsburgh from 1860 to 1940?

Pittsburgh became the meatpacking capital of the eastern United States.
Pittsubrgh became the center of the steel industry in the United States.
Pittsburgh became the most productive gold mining area in the country.
Pittsburgh became the country’s leader in telegraph and radio production.
( for my lil cousin i will give 100 point and brainlyest)