Although this depends slightly on what period you're referring to during this time, the best answer would be that "Immigrants" made up the largest percentage of workers in American industry during the late nineteenth century, since the economy had shifted towards industrial output as opposed to agrarian output.
The answer would be Latin America
Because Approximately half of immigrants living in the United States are from Mexico or other Latin American countries. Many Central Americans are fleeing because of desperate social and economic circumstances created in part by U.S. foreign policy in Central America over many decades.
#1: Houston
#2: San Antonio
#3: Dallas
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