Urine leaves the body through a sphincter in the process of urination, and muscles around the urethra, called sphincter muscles, control the flow of urine out of the body. Given these facts, what controls the urination process among the following options:
- microorganisms
 - mechanical waves
 - that material's properties
 - the looseness of soil
 - Muscles around the urethra
 - Bones around the urethra
 - The brain
 - Muscles around the anus


 

     Muscles around the urethra    