Which of the Following Describes the Principal Difference Between Menstrual

Which of the following describes the principal difference between menstrual and estrus cycles?

A) The estrus cycle occurs if the female is fertilized, whereas the menstrual cycle occurs if the female is not fertilized.
B) The menstrual cycle occurs if the female is fertilized, whereas the estrus cycle occurs if the female is not fertilized.
C) The estrus cycle lacks menstruation.
D) The menstrual cycle occurs in mammals, whereas the estrus cycle occurs in birds.
E) Only the menstrual cycle is controlled by estrogen.