You are presented with the question "what causes an object to lose energy" and the following answer choices: 
- wind and erosion
 - pressure differences.
 - refract or absorb.
 - heat is produced
 - electron transport
 - depress neuronal excitability
 - degrees Celsius
 - 2 substances rubbing together

Now knowing that friction causes an object to lose energy and friction occurs when two substances rub together, choose the best answer.


 

        2 substances rubbing together 