第30题: [单项选择]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. 参考答案:B 答案解析:[解题思路] 本题测试的是文章的细节部分。 [详细解答] “Medical researchers want to learn more about the shark's body d