(A) ** trench warfare
(B) constant movement of troops
(C) unrestricted submarine warfare
(D) the breakdown of the Russian defense
EXPLANATIONS BELOW
Concept note-1: -Trench warfare reached its highest development on the Western Front during World War I (1914–18), when armies of millions of men faced each other in a line of trenches extending from the Belgian coast through northeastern France to Switzerland.
Concept note-2: -World War I was a war of trenches. After the early war of movement in the late summer of 1914, artillery and machine guns forced the armies on the Western Front to dig trenches to protect themselves.
Concept note-3: -Trenches were common throughout the Western Front. Trench warfare in World War I was employed primarily on the Western Front, an area of northern France and Belgium that saw combat between German troops and Allied forces from France, Great Britain and, later, the United States.
Concept note-4: -During World War I, trench warfare was a defensive military tactic used extensively by both sides, allowing soldiers some protection from enemy fire but also hindering troops from readily advancing and thus prolonging the war.
Concept note-5: -Trenches provided a very efficient way for soldiers to protect themselves against heavy firepower and within four months, soldiers on all fronts had begun digging trenches. This photograph shows French infantry manning a forward line of trenches in Lorraine during January 1915.