How did the entry of the united states into world war i affect the central powers as they concentrated all their forces on the western front? a) the allied powers quickly pushed into germanyb) the central powers slowly advanced westward into francec) the central powers were unable to defeat the allied powers.

I believe the answer is C.

The entry of the US into WWI really boosted the Allied Powers' morale, while destroying the morale of the Central Powers.
While the rest of the Allies had been fighting for nearly four years at this point, when the US entered, they were fresh, energized, and idealistic.
Basically, US soldiers encouraged the other Allies and helped them get their mojo back.