Answer:
I think B because white blood cells fight off bacteria and stuff. I hope this is right for you! if not I am sorry! Have a great holiday. 
 
        
             
        
        
        
It has lily pads jat the frog jumps too and shallow water for the frog to be in the water but also away from predator fish<span />
        
             
        
        
        
Answer:
Oxygen and pyruvate.
Explanation:
The krebs cycle is a stage of cellular respiration and occurs in aerobic organisms. In this case, we can say that the Krebs Cycle only occurs in the presence of oxygen, being essential for the completion of cell respiration.
The Krebs cycle only occurs after the completion of glycolysis, as it needs to be initiated by pyruvate, which is a molecule resulting from glycolysis. In this case, in addition to oxygen, the presence of pyruvate is essential for the krebs cycle to occur.