I think it's probably learned
they need energy because growth is not the only life process that requires energy.Apart from growth, when organism reaches adulthood they need energy to aid in other processes such as locomotion,reproduction. More also, important functions such as cell replacement requires energy.
B) because it is just the equation one step ahead (if that makes sense)
Answer:
not enough clean oxygen being produced for us to breathe, loss of habitat for countless species causing them to possibly go extinct.