The primary difference between estrous and menstrual cycles is that

A) the endometrium shed by the uterus during the estrous cycle is reabsorbed, whereas the shed endometrium of menstrual cycles is excreted from the body.
B) behavioral changes during estrous cycles are much less apparent than those of menstrual cycles.
C) season and climate have less pronounced effects on estrous cycles than they do on menstrual cycles.
D) copulation normally occurs across the estrous cycle, whereas in menstrual cycles copulation only occurs during the period surrounding ovulation.
E) most estrous cycles are of much longer duration compared to menstrual cycles.