During World War I,extensive Systems of Trenches Were Used by Both

During World War I,extensive systems of trenches were used by both sides

A) because trenches prevented tanks from reaching the soldiers' positions.
B) both because the destructive power of weapons meant soldiers could not live in the open field, and because trenches prevented tanks from reaching the soldiers' positions.
C) because newly improved weaponry made conventional field battles too destructive.
D) because soldiers were safer from poisonous mustard gas closer to the ground.
E) None of these answers is correct.