a control helps to increase the reliability of the results. it allows a scientist to compare it to other group(s) in an experiment. if a scientist notices a significant different btwn the control and the other groups, he can logically conclude that the independent variable has an impact on the dependent variable. 
a variable is any factor, trait or condition that can exist in differing amounts or types. there are usually three tpes of variables in experiments: the independent, dependent and contolled. the independent variable variable is the one that is changed by the scientists to figure it's affect on something.
 
        
             
        
        
        
Answer:
1. C -make food into energy 
2. C - cellulose
3. B - organelles 
 
        
             
        
        
        
Shearing is a stress that pushes mass in two different directions. 
Hope this helped!