I think it is C, but I could be wrong because I am taking an educated guess.
<span>C) the force of gravity</span>
Nerve Regulators: Two types of nerves help to control the action of the digestive system. Extrinsic (outside) nerves come to the digestive organs from the unconscious part of the brain or from the spinal cord. They release a chemical called acetylcholine and another called adrenaline