Peristaltic Contractions,beginning at the Renal Pelvis,sweep Along the Ureter,forcing Urine

Peristaltic contractions,beginning at the renal pelvis,sweep along the ureter,forcing urine toward the urinary bladder.In a normal,healthy person,how often do these contractions occur?

A) 12-15 times a minute
B) every 30 seconds or so
C) once every minute
D) every 2 minutes
E) It is not known how often these contractions occur.