(A) poison gas
(B) airplanes
(C) submarines
(D) ** rifles
EXPLANATIONS BELOW
Concept note-1: -World War I popularized the use of the machine gun-capable of bringing down row after row of soldiers from a distance on the battlefield. This weapon, along with barbed wire and mines, made movement across open land both difficult and dangerous. Thus trench warfare was born.
Concept note-2: -Flamethrowers, poison gas, tanks, and grenades were employed extensively on the battlefields of Europe.