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