Why did general john j. pershing refuse to allow the aef to serve as "fill in the gaps" of soldiers in europe? a)because he did not want to work with the allied powers b)because he knew that a unified american front could intimidate the central powers c)because he could not speak a foreign language d)because his soldiers requested to only serve the us during selective service

He knew that a unified American front could intimidate the Central Powers.

The "American Expeditionary Forces" (AEF) were the fighting forces of American soldiers in Europe in The Great War (which we now call World War I).  According to Leatherneck magazine, published by the Marine Corps Association (September, 2017 issue), Pershing wanted the full force of the US troops to be present in Europe before deploying them "in the gaps" across various battlefields, and he believed the German soldiers (the Central Powers side) would find a unified front of American troops intimidating.  He wanted to weaken the morale of his opponents.