What key events early in 1917 combined to finally bring the united states fully into world war i?

The key events that brought the United States into World War I was Germany's unwillingness to engage in a negotiated solution, German belligerence of the seas and declaring war on all ships in European waters,German submarine attacks on U.S. ships, and German attempts to push Mexico into war with the U.S. which was discovered through the Zimmerman telegraphs. There were many factors that led the U.S. to get involved and for President Wilson to ask Congress for a deceleration of war, but these were the main factors which led to the U.S.' entry into the Great War. 

Ithink the answer is c