(mc) what effect did the entry of the united states into world war 1 have on the course of the war?

 It helped bring a quick victory to the Allies.

The United States had been providing some support to the Allies prior to its own entry into the war.  But when the US joined the effort fully with a declaration of war, that not only meant the arrival of fresh US troops but a commitment of even more American dollars to the Allies' war effort.  The American commitment greatly boosted the morale of British and French troops on the Western front and demoralized the German troops.

If you're looking at a question with other multiple choice answers, here are problems with some other answers I've seen listed with this question.

first second and third

explanation:

c. free trade can lead to low pay for workers in poorer countries.