What change occurs in the battle to allow the Americans to move forward and push the germans back?

The entry of the United States was the turning point of the war, because it made the eventual defeat of Germany possible. It had been foreseen in 1916 that if the United States went to war, the Allies' military effort against Germany would be upheld by U.S. supplies and by enormous extensions of credit.

The answer is A. Hope this helps:)