Even in fresh water sharks hunt and kill. The Thresher shark, capable of lifting a small b

oat out of the water, has been sighted a mile inland on the Fowey River in Cornwall. Killer sharks swim rivers to reach Lake Nicaragua in Central America; they average one human victim each year.

Sewage and garbage attract sharks inland. When floods carry garbage to the rivers they provide a rich diet which sometimes stimulates an epidemic of shark attacks. Warm water generally provides shark food, and a rich diet inflames the shark's aggression.

In British waters sharks usually swim peacefully between ten and twenty miles offshore where warm water currents fatten mackerel and pilchards for their food. But the shark is terrifyingly unpredictable. One seaman was severely mauled as far north as Wick in Scotland. Small boats have been attacked in the English Channel, Irish Sea and North Sea.

Most of the legends about sharks are founded in ugly fact. Even a relatively small shark——a 200 lb. Zambezi——can sever a man's leg with one bite, Sharks have up to seven rows of teeth and as one front tooth is damaged or lost another moves forward to take its place. The shark never sleeps. Unlike most fish, it has no air bladder, and it must move constantly to avoid sinking. It is a primitive creature, unchanged for sixty million years of evolution. Its skin is without the specialized scales of a fish. Fully grown, it still has five pairs of separate gills like a three-week human embryo.

But it is a brilliantly efficient machine. Its skin carries nerve endings which can detect vibrations from fish moving several miles away. Its sense of smell, the function of most of its brain, can detect one part in 600,000 of tuna fish juice in water, or the blood of a fish or animal from a quarter of a mile away. It is colour blind, and sees best in deep water, but it can distinguish shapes and patterns of light and shade easily. Once vibrations and smell have placed its prey the shark sees well enough to home in by vision for the last fifty feet. The shark eats almost anything. It will gobble old tin cans and broken bottles as well as fish, animals and humans. Beer bottles, shoes, wrist watches, car number plates, overcoats and other sharks have been found in dead sharks. Medieval records tell of entire human corpses still encased in armour.

The United States military advice on repelling sharks is to stay clothed——sharks go for exposed flesh, especially the feet. Smooth swimming at the surface is essential. Frantic splashing will simply attract sharks, and dropping below the surface makes the swimmer an easy target.

If the shark gets close, then is the time to kick, thrash and hit out. A direct hit on the snout, gills, or eyes will drive away most sharks. The exception is the Great White shark. It simply kills you.

It is less common to find sharks in ______.

A．salt water

B．fresh water

C．warm water

D．deep water