Biology of Humans 4th Edition by Judith Goodenough,Betty McGuire

Exercise 13

It is possible to swallow while standing on one's hands because
a. valves in the digestive system keep food from moving backward.
b. peristalsis pushes food along the digestive tract in the right direction.
c. bacteria clog the digestive tube and prevent food from mov- ing in the wrong direction.
d. of a wave of muscle contraction called emulsification.