(A) Armored tanks
(B) U-boats
(C) Dogfights from the air
(D) ** Trench warfare
EXPLANATIONS BELOW
Concept note-1: -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-2: -The fundamental strategy of trench warfare was to defend your own position strongly while trying to achieve a breakthrough into the enemy’s rear. The effect was to end up in attrition; the process of progressively grinding down the opposition’s resources until, ultimately, they are no longer able to wage war.