Definitely D. Because we just learned about this in science
        
                    
             
        
        
        
Can start a fire( rubbing to rocks together) and friction is used when you hit the brakes on a car( the pad pushes down on the tire). Good luck!
        
                    
             
        
        
        
It is not easily recognizable
        
                    
             
        
        
        
Answer:
Option (d).
Explanation:
Equilibrium may be defined as the state of  the equality on both the sides of the reaction. Different types of equilibrium are physical equilibrium, chemical equilibrium and dynamic equilibrium.
Chemical equilibrium may be defined as the equilibrium in which the reactants and products concentration remains constant with time. The rate of the backward reaction is equal to the rate of forward reaction.
Thus, the correct answer is option (d).