Significant Factors Leading to US Involvement in WWI

During World War I, the United States initially maintained a position of neutrality before entering the conflict. Key events and policies contributed to the shift in American public opinion and government policy. One of the major factors leading to the U.S. entry into the war was the use of unrestricted submarine warfare by Germany. Which of the following actions by Germany was a significant factor that influenced the U.S. to join the war?