题目解析：
通过听力开头就可以推测将要针对美国的秃鹰展开叙述。教授在正文中介绍了白头秃鹰作为国家象征的原因，以及国玺设计中蕴含的象征意义。这是在补充刚提出的主题。所以选D。
【对应原文】
Actually, the bald eagle has even earned itself the, um, the title of "National Emblem," which means, um, you know...it's the official symbol of the nation.
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