The third level

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Across
  1. 4. The elected head of a republic or organization.
  2. 6. Hair grown on the upper lip.
  3. 8. Your father's or mother's father.
  4. 10. a passenger compartment of a train.
  5. 11. A passageway dug through the ground, often for vehicles or trains.
  6. 14. The act of flying or a journey made by air.
  7. 15. A lamp fueled by gas, often used historically.
  8. 16. The engine that pulls a train.
  9. 18. Evidence or an argument establishing a fact.
  10. 19. A period of 100 years.
  11. 20. A flat, paper container for letters.
Down
  1. 1. A commercial activity or enterprise.
  2. 2. A system of money in general use in a country.
  3. 3. Letters or packages sent through a postal system.
  4. 5. Free from disturbance or conflict.
  5. 7. A connected series of vehicles that run on tracks.
  6. 9. The currency used in the United States and other countries.
  7. 12. Feeling sad or discontented.
  8. 13. A set of steps connecting different floors of a building.
  9. 17. The warmest season of the year.