(A) machine guns
(B) ** submarines
(C) airplanes
(D) tanks
EXPLANATIONS BELOW
Concept note-1: -On the battlefield, American forces fought using airplanes, long-range artillery, gas, motorized ambulances, mobile X-ray equipment, wireless radio, and other modern tools of war.
Concept note-2: -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-3: -Wilson cited Germany’s violation of its pledge to suspend unrestricted submarine warfare in the North Atlantic and the Mediterranean, as well as its attempts to entice Mexico into an alliance against the United States, as his reasons for declaring war.