到了本节目的最后一阶段了，非常感谢大家一直一来的支持，要完结了还真的舍不得呢！
希望大家能坚持把这最后的模拟题听完，偶在这里就先预祝今年参加专四考试的同志们能有个好成绩！
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defense system
About 40% 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 diseases. They know that sharks recover quickly from injuries. Sharks appear never to suffer infections, cancer or heart disease.