(1) D. Fighting in the Easter Front was mobile, while the Western Front devolved into a stalemate because of the use of trench warfare.
(2) C. U-boat attacks destroyed convoy ships and hampered supply lines, which affected the armies' ability to adequately feed and supply their soldiers.
(3) A. Poison gas was a innovation used by the Germans to try to break the stalemate on the Western Front. After the war it was forbidden.