Fact 1: Muscle contraction occurs when muscle fibers get shorter.

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

Given the two facts above, answer the question "what contracts when fibers get shorter?" with the following options: 
- strength
 - strong
 - h2o
 - bushes
 - density
 - wheat
 - firm hugs
 - biceps


 

        biceps 