(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.

crop rotation kepy different plants in the soil instead of once constintly. when switching out plants you have different nutrients being absorbed at different speeds and different roots growing differently therfore the spoil and water flow have to adapt and carry fresh nutrients to carry the constant changes that were going on in the soil.