What is this passage mainly about?

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问题 What is this passage mainly about?
  
Scientists say sharks have lived in the world’s oceans for millions of years. Scientists say there are more than 350 different kinds of sharks.
     Sharks do not have bones, and a shark has an extremely good sense of smell. It can find. small amounts of substances in the water, such as blood, body liquids and chemicals produced by animals. Sharks also sense electrical and magnetic power linked to nerves and muscles of living animals. These powerful senses help them find their fond. Some sharks will eat just about anything. Many unusual things have been found in the stomachs of some large sharks. They include shoes, dogs, a cow’s foot and metal protective clothing.
     About forty percent of the different kinds of sharks lay eggs. The others give birth to live young. Some sharks carry their young inside their bodies like humans do.
     Scientists are beginning to understand the importance of sharks to humans. Medical researchers want to learn more about the shark’s body defense system against disease. They know that sharks recover quickly from injuries. Sharks appear never to suffer infections, cancer or heart diseases.

选项 A、For their tasty meat.
B、For the principle of their defense system against disease.
C、For their solid skin.
D、For the principle of their birth modes.

答案B

解析 根据文中提到的“...the shark’s body defense system against disease.”可知答案为B 。
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