Find out the key word

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Across
  1. 4. Power used for light, heat and machines
  2. 7. A very large area of salt water
  3. 10. The natural world, including plants, animals and landscapes
  4. 11. Things that people throw away because they do not want them anymore
Down
  1. 1. A car, bus or truck is a type of this
  2. 2. Related to the sea and the animals or plants living in it
  3. 3. A problem or an important topic that people discuss
  4. 5. Unwanted or unpleasant sound
  5. 6. To use waste materials again instead of throwing them away
  6. 8. Existing in or related to nature
  7. 9. The planet where we live