What event was the catalyst for the United Stated to entered the war

answer:

Two events forced the U.S. to join the war. The first was Germany's unrestricted submarine warfare. On May 7, 1915, a German submarine sunk the British ocean liner RMS Lusitania, killing 1,198 passengers including 128 Americans. This was the catalyst that changed public opinion on America's neutrality.Jan 17, 2018

Explanation:

its c not d   tell me if that´s correct