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阅读下面的短文，文中有15处空白，每处空白给出了4个选项，请根据短文的内容从4个选项中选择1个最佳答案。 Sharks Perform a Service for Earths Waters It is hard to get people to think of sharks as anything but a deadly enemy. They are thought to _____51 people frequently. But these fish perform a_____ 52 service for earths waters and for human beings. Yet business and sport fishing are threatening their _____53 Some sharks are at risk of disappearing from_____ 54 。 Warm weather may influence both fish and shark activity. Many fish swim near coastal areas _____55 their warm waters. Experts say sharks may follow the fish into the same areas， _____56 people also swim. In fact， most sharks do not purposely charge at or bite humans. They are thought to mistake a person _____57 a sea animal， such as a seal or sea lion. That is why people should not swim in the ocean when the sun goes down or comes up. Those are the _____58 when sharks are looking for food. Experts also say that bright colors and shiny jewelry may cause sharks to attack. A shark has an extremely good sense of smell. It can find small amounts of substances in water， such as blood， body liquids and _____59 produced by animals. These powerful _____60 help sharks find their food. Sharks eat fish， any _____61 sharks， and plants that live in the ocean. | 
