What was one of the events that led to u. s. involvement in wwi

First event leading to US's enter into WWI:Sinking of the Lusitania.
May 7, 1915.
American gov said stop unrestricted warfare.
Germans said okay.

Second event leading to US's enter to WWI:Germany publicly announces using unrestricted sub warfare again.
January 1917.
America gives second warning.
Germany ignores warning.

Third event leading US's enter into WWI:Zimmerman.
Telegram sent to mexico by germany.
Intercepted and decoded by great britain.
translation given to US in Feb. 1917
America begins to mobilize for war.

Fourth event leading to US's entrance in WWI:April 1917.
German U-boats sink four US merchant ships.
America enters the war April 6 1917.

The answer is d.  the north had organized schools, whereas the south had few organized schools.