How is oxygen created?

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Across
  1. 1. 5
  2. 5. 6
  3. 8. 2
  4. 10. 3
Down
  1. 2. 9
  2. 3. 8
  3. 4. 1
  4. 6. 4
  5. 7. 10
  6. 9. 7