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