Answer:
A  theory
Explanation:
A theory is an idea used to account for a situation or justify a course of action.
 
        
             
        
        
        
Keystone predators effect their habitat by keeping the population of animals down.
        
             
        
        
        
Polar because it always is below 32 degrees and has no rainfall
        
                    
             
        
        
        
Answer:
for 16 it is heart. 
Explanation:
for 17 it is kidney to bladder