Which Statement About the Reproductive Cycles of Dogs Is True

Which statement about the reproductive cycles of dogs is true?

A) They undergo menstruation when a blastocyst fails to implant.
B) They exhibit uterine bleeding similar to menses.
C) They are sexually receptive in between each ovarian cycle.
D) They are sexually receptive during a restricted period of time called estrus.
E) They lack uterine cycles.