31. Astronauts in zero gravity are able to move food through their digestive tracts because __________.

A. smooth muscle produces waves of contractions that propel material through the digestive tract B. skeletal muscle puts pressure on the digestive tract C. cardiac muscle maintains a high pressure that moves material through the digestive tract D. striated muscle creates a pressure gradient that forces material from one end of the digestive tract to the other E. skeletal muscle sphincters contract and allow materials to move through the digestive tract