The answer is iron. Research has shown an increase in serum iron concentreation in athletes and a concomitant decrease in magnesium ions. The increase in iron concentration is thought to be attributed to tissue injury from the strenuous physical activity. The decrease in magnesium is due to demand by the skeletal muscles. 
        
             
        
        
        
The variables are defined as the factors in an experiment, which are liable to change. The variable can be dependent or independent. The Independent variable is a variable, which is unaffected by the changes in the other variables. 
In this case, the brand of the food Shara feeds to the rats is independent factor, but it is changes in the system, so, it is an independent variable. 
The effect on the weight of the rat is dependent on the brand of food they are consuming. So, the weight of rats is dependent variable. 
 
        
                    
             
        
        
        
A substrate is a reactant that is catalyzed by an enzyme.
        
                    
             
        
        
        
Absorbing other cells gave a competitive advantage.
Hope this helps!<span />
        
             
        
        
        
Answer:
single-stranded RNA genome I think idrk