陜西自考網(wǎng)> 試題題庫列表頁> Sharks Perform a Service for Earth's WatersIt is hard to get people to think of sharks as anything but a deadly enemy. They are thought to attack frequently. Although these fish perform a 41___ ( value) service for earth's waters and for human beings. business and sport fishing are 42___ (threat) their existence.As a result,some sharks are at risk of disappearing from Earth.Warm weather may influence. both fish and shark 43___ (active). Many fish swim near coastal areas because of their warm waters. Experts say sharks may follow the fish into the same areas，where people also swim. In fact ,most sharks do not44___( purpose)charge at or bite humans. They are__ 45 ( think) to mistake a person for a sea animal,such as a sea (海豹) or sea lion, That is why people should not swim in the ocean when the sun.46___ (go) down or comes up. because those are the times when sharks are looking for food. Experts also say that bright colors and shiny jewelry(珠 寶) may cause sharks to attack.Medical researchers want to learn 47___ (much) about the shark's body defense and immune(免疫的) systems against disease. 48__ ( research) know that sharks recover quickly from injuries. They study the shark in hope of finding a way to fight human disease.Sharks are important for the world's oceans,as they eat 49___(injure). and diseased fish. Their 50__ ( hunt) activities mean that the numbers of other fish in ocean waters do not become too great.This protects the plants and other forms of life that exist in the oceans.