共用题干
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 to1people frequently. But these fish perform a2service for earth's waters and for human beings. Yet business and sport fishing are threatening their3.Some sharks are at risk of disappearing from4.
Warm weather may influence both fish and shark activity. Many fish swim near coastal areas5their warm waters. Experts say sharks may follow the fish into the same areas,6people also swim. In fact,most sharks do not purposely charge at or bite humans. They are thought to mistake a person7a 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 the8 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 wa-ter,such as blood,body liquids and9 produced by animals. These powerful10help sharks find their food. Sharks eat fish,any11sharks,and plants that live in the ocean.
Medical researchers want to learn more about the shark's body defense,and immune 12against disease. Researchers know that sharks13quickly from injuries. They study the shark in hopes of finding a way to fight human disease.
Sharks are important for the world's14.They eat injured and diseased fish. Their hunting activities mean that the numbers of other fish in ocean waters do not become too15．This protects the plants and other forms of life that exist in the oceans.
9._________
A: sciences
B: mathematics
C: chemicals
D: physics

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