4. Fill in the appropriate word from the list. 1. natural…. 2. industrial…. 3. harmful…. 4. oxygen…. 5. rare…. 6. ozone…. 7. ecological…. 8. acid…. 9. air…. 10. dead…. 11. beautiful….. substances, species, riches, security, fish, layer, rain, enterprises, balance, pollution, forests