During WW1, extensive systems of trenches were used by both sides
A. because the destructive power of weapons ment soldiers could not live in the open field
B. because they prevented tanks form reaching the soldiers' positions
C. because the soldiers were safer from poisonous mustard gas closer to the ground
D. both because the destructive power of weapons ment soldiers could not live in the open field and because they prevented tanks form reaching the soldiers' positions
E. None of the above
Answer: A