填空题：回答题Sharks Perform a Service for Earth's Waters　　It is hard t

回答题
Sharks Perform a Service for Earth's Waters
　　It 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,busind sport fishing are __42__ (threat) their existence As a result, some sharks are at risk of disapearing 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 not __44__ (purpose) charge at or bite humans. They are __45__ (think) to mistake a person for a sea animal, such as a seal（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 (珠宝)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.
请在第__41__处填上正确答案。