完形填空
　　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 Earth’s 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 earth’s 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.
　　Medical researchers want to learn more about the shark’s body defense and immune _____62 against disease. Researchers know that sharks _____63 quickly from injuries. They study the shark in hopes of finding a way to fight human disease.
　　Sharks are important for the world’s _____64 . They eat injured and diseased fish. Their hunting activities mean that the numbers of other fish in ocean waters do not become too _____65 This protects the plants and other forms of life that exist in the oceans.
　　练习：
　　51. A attack B meet C love D visit
　　52. A terrible B eatable C valuable D possible
　　53. A source B existence C friends D fish
　　54. A Space B Sky C Land D Earth
　　55. A because B since C because of D by reason that
　　56. A whose B which C that D where
　　57. A to B for C like D with
　　58. A times B places C seas D oceans
　　59. A sciences B mathematics C chemicals D physics
　　60. A feelings B senses C touches D tastes
　　61. A those B these C another D other
　　62. A systems B processes C ideas D circles
　　63. A recover B reform C return D rely
　　64. A rivers B oceans C forests D mountains
　　65. A weak B little C few D great

　　【参考答案】 ACBDC DBACB DAABD