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

Fact 2: Exercise causes muscles to contract.

Given the two facts above, answer the question "What causes muscle fibers to get shorter?" with the following options: 
- friction
 - acetic acid
 - heat or cold
 - sweating
 - electrically
 - resistance
 - Damages them
 - exercise


 

        exercise 