Final answer:
Most immigrants to the United States since 1945 have come from various regions, with Europe being the largest source of immigrants in the early 20th century, but declining over time. In the 2000s, immigration from the rest of the Americas, particularly Mexico, and Asia became more predominant.
Step-by-step explanation:
Since 1945, most immigrants to the United States have come from different regions. In the first decade of the twentieth century, over 90% of the immigrants were from Europe. However, by the end of the century, less than 20% of the total immigrants were from Europe. In the 2000s, about half of the U.S. immigration came from the rest of the Americas, especially Mexico, and approximately a quarter came from various countries in Asia.