D.
variable-ratio schedule of reinforcement.
Just as the organs in an organ system work together to accomplish their task, so the different organ systems also cooperate to keep the body running. For example, the respiratory system and the circulatory system work closely together to deliver oxygen to cells and to get rid of the carbon dioxide the cells produce.
are you ok? like seriously is your home life ok i dont want you to be going through anything you shouldnt.
D - spine alignment and foot size.
Late australopiths had an S-shaped spine that allowed for balance when standing. Shorter and less flexible toes made pushing off the ground much easier.
