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Questions 17--20 are based on the following passage. You now have 20 seconds to read Questions 17--20. 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--20 are based on the following passage. You now have 20 seconds to read Questions 17--20. Why sharks are useful for humans 您的浏览器不支持 audio 元素。