Normally menstruation occurs when . A) blood levels of FSH fall off
B) blood levels of estrogen and progesterone decrease
C) blood levels of estrogen and progesterone increase
D) the corpus luteum secretes estrogen

B) blood levels of estrogen and progesterone decrease

Explanation:

Menstruation is a normal phenomenon in a fertile female. It is the biological process whereby the flow of blood from the uterine lining occurs. Menstruation usually occurs after implantation, which is the attachment of a fertilized egg in the uterus, fails to occur.

On a monthly basis, the female reproductive organ prepares the uterus for implantation, which is physiological stimulated by an increase in the blood levels of estrogen and progesterone. However, when there is no implantation, the level of these two reproductive hormones (estrogen and progesterone) fall, and menstruation is stimulated to occur.

oxygen +1 most likely