World War One Crossword puzzle

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Across
  1. 2. used by all soldiers in WW1 (5)
  2. 3. area between opposing front lines
  3. 6. need cutters to get through this (6,4)
  4. 8. tanks were first used at this battle (5)
  5. 12. the back wall of a trench (7)
  6. 14. this gas was used at Ypres in 1915 (8)
  7. 18. this colorless gas is deadly (7)
  8. 20. trench between front & support trench (13)
  9. 21. get on this to shoot (Two words 4,4)
Down
  1. 1. wood at the bottom of a trench (4,6)
  2. 4. this gun needed 4-6 men to work it (7)
  3. 5. this american liner was blown up (9)
  4. 6. trench hiding place (4,4)
  5. 7. caused by standing in water (6,4)
  6. 9. used for protection (4,3)
  7. 10. these weapons were used by submarines (9)
  8. 11. the front wall of a trench (7)
  9. 13. also known as Blimp (8)
  10. 15. name of the first tank little (6)
  11. 16. a fight between 2 planes (8)
  12. 17. underground storage area(6)
  13. 19. drainage channel (4)