Answer:
c lower oxygen levels
Explanation:
the higher up the person is the closer to the atmosphere so there is less oxygen
 
        
             
        
        
        
Its because a long spine cacti can reproduce more because it has more survival capacity than the short spine cacti and because of this reason the short spine cacti cannot protect it self because it has a shorter spine that could protect itself from danger. 
        
                    
             
        
        
        
Hello!
Carrying capacity, in biology, is the mps (maximum population size) of the species that the environment is able to take care of.  The environment should be able to give food, habitat, and water, and other necessities needed for the certain animal species. If there are to many of the species, the environment may not be able to care for all animals, due to the fact that there will not be enough resources for all of the animals.
Hope this helps, have an awesome day/night! :)
 
        
             
        
        
        
Reading background information because it gives ideas from the author himself.
        
                    
             
        
        
        
They train our immune system so it's ready when our bodies are attacked, and they aid in digestion and supply us with vitamins. ... Scientists and doctors can even utilize prokaryotes to help the human body.