Fact 1: Muscles can only contract.

Fact 2: Muscles in the upper arm called biceps are used to flex the arm.

Given the two facts above, answer the question "what can only contract?" with the following options: 
- nerves
 - Epidermis
 - the skin
 - strong
 - biceps
 - Bend
 - lungs
 - forces


 

     biceps    