Read the following passages and answer the items that follow.

Passage:

On a surface which is free from obstacles, such as a clear road or a path, only two or three species of snakes can hope to catch up with a human being, even if they are foolish to try. A snake seems to move very fast but its movements are deceptive. In spite of the swift, wave-like motions of its body, the snake crawls along the ground at no more than the speed of man's walk. It may, however, have an advantage inside a jungle, where the progress of a man is obstructed by thorny bushes. But in such places, the footsteps of a man are usually more than enough to warn snakes to keep away. Although they have no ears of the usual kind, they can feel slight vibrations of the ground through their bodies, and thus get an early warning of danger.

Q: What helps the snakes to receive advance warning is their sensitivity to: