The better answer is:
C) Miles of trenches dug to defend positions.
There was use of poison gas attacks, but that strategy began after the employment of trench warfare and because of the situation of trench warfare. The deployment of poison gases was an attempt to demoralize their opponents because they couldn't find any other way to mount a successful attack. Trench warfare in World War I meant that the armies dug into trenches to hold their ground. But it proved impossible for them to make any advances against each other. If they would try to mount an attack, venturing into "no man's land" between the trenches, they'd get mowed down by machine gun fire. So n addition to bullets and artillery fire, trench warfare also came to involve the use of chemical weapons like mustard gas. But trench warfare itself was the dominating reality of military strategy in World War I, and a strategy which led to the use of chemical weapons (poison gases).
