What Changed the Course of World War I in the Spring

Multiple Choice

What changed the course of World War I in the spring of 1917?

A) The use of machine gun and artillery in trench warfare.
B) The collapse of Russia and the U.S.entry into the war.
C) The withdrawal of Turkey from the war effort.
D) Unexpected diplomatic efforts by the Japanese.
E) The German's completely isolating the British Isles by submarine warfare.