A、It tries to categorize the different kinds of sharks throughout the world. B、It tries to warn humans of the dangers posed by s

Most people picture sharks as huge, powerful, frightening predators, ready at any moments to use their sharp teeth to attack careless swimmers without provocation. But, people may have some wrong ideas on the conception of sharks.
    First, there are 350 species of shark, and not all of them are large. They range in size from the dwarf shark, which can be only 6 inches long and can be held in the palm of the hand, to the whale shark, which can be more than 55 feet long.
    Second, the different species of shark vary tremendously in the number and type of teeth. A shark can have from one to seven sets of teeth at the same time, and some types of shark can have several hundred teeth in each jaw. It is true that the fierce and predatory species do possess extremely sharp and brutal teeth used to rip their prey apart: many other types of shark, however, have teeth more adapted to grabbing and holding than to cutting and slashing.
    Finally, not all sharks are predatory animals ready to strike out at humans. In fact, only 12 of the 350 species of shark have been known to attack humans, and a shark needs to be provoked in order to attack. The types of shark that have the worst record with humans are the tiger shark, the bull shark, and the great white shark. However, for most species of shark, even some of the largest types, there are no known instances of attacks on humans.