One hint is that, if a message is being sent, the "sender" is probably the nervous systems. There are several subsystems to the nervous system, but it regulates and controls virtually all bodily functions. The nervous system monitors your body's temperature, and takes actions to regulate it, such as telling your muscular system to shiver to generate more heat. It sends this message to the muscular system because your muscles cause bodily movement, such as walking, lifting weights, and even shivering.
When your nervous system determines that your body temperature has returned to normal, it again sends a message to your muscular system to stop shivering.
B. Mutations cause a person's body cells to divide uncontrollably. There isn't a signal for cells to stop dividing, so this can cause cancer because cancer cells won't stop dividing without this signal.
