Urine leaves the body through a sphincter in the process of urination, and urination requires relaxation of the sphincter muscle. Given these facts, what does the process of urination require among the following options:
- aerobic capacity
 - relaxation of muscles
 - the looseness of soil
 - sunlight
 - air
 - metabolic reaction
 - blood
 - Converting waste.


 

  relaxation of muscles       