单项选择题

Questions 17 to 20 are based on the following passage. At the end of the passage ,you will be given 20 seconds to answer the questions.
Now listen to the passage.
Why sharks are useful for humans

A．Their tasty meat.
B．The principle of their defense system against disease.
C．Their solid skin.
D．The principle of their birth modes.

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单项选择题

单项选择题

Questions 17 to 20 are based on the following passage. At the end of the passage ,you will be given 20 seconds to answer the questions. Now listen to the passage. Why sharks are useful for humans 您的浏览器不支持 audio 元素。