The swallowed food moves downwards in the alimentary canal because of
- force provided by the muscular tongue.
- the flow of water taken with the food.
- gravitational pull.
- the contraction of muscles in the wall of food pipe.
As soon as the food enters the food pipe, the muscles in its walls start alternate contractions and relaxation producing a wave-like movement called peristaltic. These movements are essential for pushing the food downwards in the alimentary canal.
The correct option is D.