You are presented with the question "Many muscle fibers contracting at once can be induced by" and the following answer choices: 
- exposed genitalia
 - exposure to heat
 - sweating
 - exposed biceps
 - mechanical waves
 - Electrical energy
 - exposure to cold
 - Veins and arteries.

Now knowing that shivering occurs when many muscles contract a little bit all at once and another response to cold is shivering, choose the best answer.


 

       exposure to cold  