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  3. There are about 350 species of sharks. The largest sharks are the whale shark and the basking shark, which feed on plankton and

There are about 350 species of sharks. The largest sharks are the whale shark and the basking shark, which feed on plankton and

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问题 There are about 350 species of sharks. The largest sharks are the whale shark and the basking shark, which feed on plankton and small fish. The adult whale shark grows to about 13 meters long, while the basking shark extends a little over 8 meters.(46)On the other hand, there are a number of them which have attacked and killed swimmers. These sharks include the great hammerhead, blue, whitetip, and, of course, the great white shark.
   (47)The great white grows to between 5 and 8 meters long and can weigh more than 1300 kilograms. It can swim through the eater at speeds reaching 32 kilometers per hour. However, one of the fastest swimmers in the shark family is the blue shark, which can swim at a speed of nearly 752 kilometers per hour.
   (48)They can grow as long as 5 centimeters, and they are very sharp. Unlike humans, who have two sets of teeth in their life time, some sharks can have as many as 5 or 6 sets. It is rare for a shark to have fewer than 4 sets. It is rare for a shark to have fewer than 4 sets. As the shark’s teeth are worn out, they fall out and are replaced by the next row.
   (49)In a lifetime some sharks can grow and lose between 10,000 and 30,000 teeth.
   Sea biologists have discovered that the shark has a remarkably large brain and a well-developed sensory system. The animal has an extraordinary sense of smell and excellent vision.
   (50)Special sensory pits on the shark’s nose and chin can detect weak electrical fields in the water that are produced by fish and other animals. Scientists have concluded that sharks associate food with electrical fields. Scientists also believe that sharks create their own electrical fields to aim them in knowing where to go in the sea.

     A. Must of the species are not particularly cruel.
     B. Sharks have lived on earth for more than 180 million years.
     C. The great white is the largest and cruelest of all animals known to attack humans.
     D. Some sharks can wear out a full set of teeth in less than 6 months.
     E. One of the most unusual aspects of the shark is its teeth.
     F. But sharks are also sensitive to electrical fields.
选项
答案D
解析 上文说当鲨鱼的牙齿磨损时，就会脱落而由下排牙齿来代替。下文说，终其一生一些鲨鱼生长和脱落的牙齿数可达10 000～30 000，可以推知鲨鱼的牙齿磨损得很快，故将D句“一些鲨鱼不出6个月就能磨坏一副牙”放入此处。
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