The most accurate statement about the United States' involvement in World War I is:
The United States stayed out of the war for more than two and a half years.
The U.S. entered the war in April 1917, while the war began in July 1914, meaning it was officially neutral for over two and a half years. The statement regarding heavy casualties is inaccurate since the U.S. became involved later in the conflict and did not suffer the same level of casualties as some European nations.