Fact 1: Shivering occurs when many muscles contract a little bit all at once.

Fact 2: Involuntary contractions, shivering, cause the production of heat.

Given the two facts above, answer the question "What occurs when many muscles contract a little bit all at once?" with the following options: 
- the production of cells
 - preventing heat transfer
 - To keep their body heat.
 - the production of heat
 - Generating heat
 - recovery time from fatigue
 - the production of cold
 - the production of skin


 

    the production of heat     