Fact 1: Urine leaves the body through a sphincter in the process of urination.

Fact 2: Urination requires relaxation of the sphincter muscle.

Given the two facts above, answer the question "What does the process of urination require?" with the following options: 
- aerobic capacity
 - relaxation of muscles
 - the looseness of soil
 - sunlight
 - air
 - metabolic reaction
 - blood
 - Converting waste.


 

  relaxation of muscles       