According to the passage, one scientific assumption about elephants is that ----.

Elephants have to keep in touch across large
distances. Even when they are out of hearing range
of one another, in forests or the great plains of East
Africa, they are often spotted moving in the same
direction. Sometimes they seem to stand still in their
tracks and move their feet up and down, which leads
some scientists to believe they have sensitive cells in
their feet. Such cells would enable them to pick up
low frequency vibrations from the ground, waves that
travel distances of up to 16 km.