(A) ** Trench warfare
(B) U-Boats
(C) Stalemate
EXPLANATIONS BELOW
Concept note-1: -What is trench warfare? Trench warfare is a type of combat in which the opposing sides attack, counterattack, and defend from relatively permanent systems of trenches dug into the ground.
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. Fighting ground to a stalemate.
Concept note-3: -Trenches-long, deep ditches dug as protective defenses-are most often associated with World War I, and the results of trench warfare in that conflict were hellish indeed.
Concept note-4: -The embanked rear lip of the trench was called the parados, which protected the soldier’s back from shells falling behind the trench. The sides of the trench were often revetted with sandbags, wire mesh, wooden frames and sometimes roofs. The floor of the trench was usually covered by wooden duckboards.