(A) Airplanes and U-boats
(B) ** Tanks and airplanes
(C) Hand grenades and U-boats
(D) Mines and hand grenades
EXPLANATIONS BELOW
Concept note-1: -The addition of the American troops broke the stalemate and pushed the Germans back into Germany, forcing them into the armistice that ended the devastation of the First World War.
Concept note-2: -These new tanks, built to break the stalemate of trench warfare, were unlike anything in use today. They were long and rhomboidal in shape with tracks encircling the body to aid in crossing deep and wide trenches.
Concept note-3: -Tanks. Tanks were developed by the British Army as a mechanical solution to the trench warfare stalemate.
Concept note-4: -The Allies’ increased use of the tank in 1918 marked the beginning of the end of trench warfare, however, since the tank was invulnerable to the machine gun and rifle fire that were the trenches’ ultimate defense.