I'm not terrific at history, but I believe it was because the Americans were fresh troops and they weren't worn down by years of trench warfare.
I believe the answer would be D.
Because they wanted to go to war
People from the Eastern, such as slavic-speaking regions, and Southern European regions, such as Greece and Italy, made up the majority of immigrants from the 1890s to the 1920s. This marked a shift from the previous decades, in which Central and Northern European regions were sending the most immigrants to the United States.