The menstrual flow is the result of
a. no implantation of a zygote.
b. decreased levels of progesterone.
c. discarded uterine linings.
d. no implantation of a zygote, discarded uterine linings, and decreased levels of progesterone.

. D.no implantation of a zygote, discarded uterine linings, and decreased levels of progesterone.

Explanation:

It is the regular discharge (monthly) of blood and mucosal tissue from the endometrium in females..its marks the starts of puberty in females. The first  blood shed is called Menarche. It occurs between the age 12 and 15 years.

The collapse of the endometrium  occurs when the levels of progesterone drops because Implantation failed to occurs. (progesterone is a pregnancy hormone secreted by the corpus luteum in the ovaries  during ovulation It maintains the growth of the blood vessels supplying the endometrium.

Therefore if implantation failed, coupus luteum breaks down, the levels of progesterone drops. Since endometrium  is no longer maintained by the progesterone; it collapsed . this is menstruation.

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