Question

By 1917, the countries fighting in World War I had reached a stalemate. A stalemate is a situation where no one can make progress. What happened in 1917 that ended the stalemate?
the invasion at Normandy
the use of mustard gas
the use of submarine warfare
the U.S entry into the war

Answer 1

I do believe its The U.S entry of war, I hope this helps!!  :D

Answer 2

Study Island answer: The United States entry of war