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Sharks have been around for a very long time．It means that they have been on earth for thousands of years，and have changed little．A shark is a fish，but it has many （71） 　　from a fish．For example，if a shark （72） 　　swimming，it will sink（下沉）．

There are （73） 　     　200 and 250 kinds of sharks in different size．The largest can be （74） 　　long as 15 meters．Sharks feed on fish，（75） 　　sea animals，or smaller sharks．It is said that one of the most （76） 　　sharks is the Great White shark．They have attacked（攻击）a number of people off the beaches of America，Australia，New Zealand and South Africa also have shark attacks．In some places there （77） 　 　watchtowers on the beach to warm people about sharks in the water．（78） 　 　a shark sppears，a bell rings to tell people to get out of the water．Shark attack about 100 people a year in America．Perhaps the shark（79） 　　thinks that the people is a kind of sea animal，or （80） 　　good to eat．