到了本節(jié)目的最后一階段了，非常感謝大家一直一來(lái)的支持，要完結(jié)了還真的舍不得呢！

希望大家能堅(jiān)持把這最后的模擬題聽完，偶在這里就先預(yù)祝今年參加專四考試的同志們能有個(gè)好成績(jī)！

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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.