When did oxygen levels start to decline from 35 percent down to today's levels during what period

Wind energy is plentiful in regions with vast open spaces it is a non polluting renewable resource what could be an economic factor that makes the use of this resource less feasible then non renewable resource even in such regions

This is the diveristy of ecosystems natural communites and habits it is the variety of ways that species interact with each other and their environment

18. which statement about cancer is true?