Final answer:
The entry of the United States into World War I boosted the morale of the Allied soldiers and demoralized the German troops.
Step-by-step explanation:
The statement that best describes the effect of the United States entering World War I is: The arrival of U.S. troops boosted the morale of tired Allied soldiers and demoralized German troops. When the U.S. entered the war in 1917, it brought fresh troops, resources, and supplies to the Allies, which revitalized their fighting spirit. This, in turn, had a demoralizing effect on the German troops, who now had to face a stronger and more determined opposition.
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