which of the following contributes to the difficulty of the human birth process compared with other primates? a. humans have lost the instincts that support the birth process. b. other primates usually produce twins or triplets. c. the human pelvis has been shortened to better support bipedal locomotion. d. humans brains grow at a faster rate in the womb than they do in other primates. e. human newborns are immature and so do not contribute to their own birth.