Across
- 5. to be maintained
- 8. group of babies(cat)
- 10. branch of biology studying organisms
- 12. layer of gas around the earth's surface
- 13. place full of trash
- 14. not alive anymore
- 16. derived from living matter
- 18. to keep something safe
- 20. layer above the earth
Down
- 1. biological community
- 2. to use old things to make new
- 3. to make foul or unclean
- 4. production of discharge
- 6. surrounding conditions in which a person lives
- 7. able to use again __ energy
- 9. to collect objects that can be used again
- 11. place where animals live
- 15. to save something
- 17. converting organic materials into nutrients
- 19. sun