Tanks · Machine guns
· Poison gas
· Flamethrowers
· Airplanes
These new military technologies resulted in a huge expansion of the military tactic known as trench warfare and made the most deadly war up to that point in history.

a.
Vietnam
b.
World War I
c.
World War II
d.
American Civil War

The correct answer is B. World War I.

Explanation:

A trench warfare is a war in which both sides have dug themselves opposite each other in trenches and other fortified positions, with the aim of halting the advance of the enemy, which culminates in a stalemate where neither side succeeds in breaking through the enemy lines. The best-known trench warfare is World War I.

In general, trench warfare is the result of technological advances that allow enemy soldiers to be killed faster than they can advance. For example, in World War I, repeating rifles, machine guns, and modern artillery made it possible to mow down soldiers in the open by bushes. However, as armies still moved largely on foot and horseback, they could not cross those same open battlefields fast enough to get close enough to the enemy and fire effectively at the enemy.