单项选择题

TEXT B
One of the mysteries of nature is animal communication. Creatures of the sea move through their liquid element without any hearable sound. Yet they are able to sense the silent passing or attack of others of their kind. This is because bodies moving in water create a pressure wave, something like a gust of wind produced by a passing automobile. A narrow sensor system running along each side of the shark, from eye to tail, receives and interprets these pressure waves. I have seen sharks appear swiftly, obviously drawn by the motion of a diver’s hands, which had sent out strong waves.
One might think it impossible to distinguish odors in water. Yet sharks, surprisingly, are able to follow a scent across miles of ocean and arrive at its exact source. A diver who spears a fish and attaches its bloody body to his belt, and then goes on with the hunt, becomes a natural prey to the shark. I have seen sharks follow a scent exactly like the dogs of a hunting pack. It is no wonder that the Greeks gave them the name "hounds of the sea."
One of the most widely believed--and dangerous--legends about the shark is that he has poor eyesight. On the contrary, the shark is well equipped to see at a distance and to distinguish among forms. This fact was proved to me one day when I went into the water off the coast of Africa. I sighted a shark at some distance from me as I was floating at a very shallow depth. Since I was making no movement, the sound of bubbles from my diving apparatus would be confused with the light splashing of the water on a rock. I turned my eyes away for a few seconds, to study the design of a giant ray just beneath me. I am not sure now whether it was simple instinct or a feeling of movement, but I turned back abruptly toward the location of the shark. And, immediately, every muscle of my body tensed. He was no more than 30 feet away, launched toward me as hard and swift as a missile.
The sight of a shark coming at you head-on is strange. Obviously it is from this angle that he seems most frightening--the very symbol of evil, with the half-opened mouth and the three regularly spaced fins (鳍). When the shark had approached to within two feet of the rubber fins I had thrown at him as a gesture of self-defense, he turned suddenly and swam back toward the depths. There had been no sound, no scent. It appears certain that sight alone was responsible for this approach.
The Greeks call sharks "hounds of the sea" because ______.