Which Statement Describes the Menstrual Cycle

Which statement describes the menstrual cycle?

A) Ovulation does not have to occur in every menstrual cycle,especially for girls in puberty and older women.
B) Almost all women come close to approximating the "perfect" 28-day menstrual cycle.
C) Irrespective of the duration of one's menstrual cycle,ovulation invariably occurs on the 14thday from the start of menstruation.
D) The follicular phase of the menstrual cycle is the most constant,extending for 10 days from ovulation to menstruation.