Why does the microbiota of the reproductive tract change as the female ages from childhood to adulthood to old age?

A) Her estrogen and progesterone levels change over time.
B) The pH of the vagina changes over time.
C) The microbes in the vagina change the environment, which affects the populations of microbes living there.
D) Post-menopausal women have a stable microbiota.
E) All of these statements are true.