menstrual period

[RADIOhead]

toward the end of the menstrual period (ovarian) cycle, retention of the thickened endometrium of the uterus depends upon the
A. egg being fertilized and the progesterone level remaining high
B. decrease of gonadotrophin production by the ovaries
C. egg being fertilized and the progesterone level decreasing
D. increase of gonadotropin production by the ovaries