Question 23 is based on the recording you have just heard.

听力原文

    Virtually every American can recognize a dollar bill at a mere glance. Many can identify it by its sound or texture. (22) [But few people indeed can accurately describe the world’s most powerful, important currency.] 

    The American dollar bill is colored with black ink on one side and green on the other; (23) [the exact composition of the paper and ink is a closely guarded government secret.] Despite its weighty importance, the dollar bill actually weighs little. It requires nearly 500 bills to tip the scales at pound. Not only is the dollar bill lightweight, but it also has a brief lifespan. Few dollar bills survive longer than 18 months.

    The word “dollar” is taken from the German word “thaler”, the name for the world’s most important currency in the 16th century. The thaler was a silver coin first minted in 1518 under the reign of Charles V, Emperor of Germany.

    The concept of paper money is a relatively recent innovation in the history of American currency. When the Constitution was signed, people had little regard for paper money because of its steadily decreasing value during the colonial era. (24) [Because of this lack of faith, the new American government minted only coins for common currency.] Interest-bearing bank notes were issued at the same time, but their purpose was limited to providing money for urgent government crises, such as American involvement in the War of 1812. The first non-interest bearing paper currency was authorized by Congress in 1862 at the height of the Civil War. At this point, citizens’ old fears of devalued paper currency had calmed, and the dollar bill was born. The new green-colored paper money quickly earned the nickname “greenback”.

    Today, the American dollar bill is a product of the Federal Reserve, and is issued from the 12 Federal Reserve banks around the United States. The government keeps a steady supply of approximately 2 billion bills in circulation at all times.

    Controversy continues to surround the true value of the dollar bill. (25) [American history has seen generations of politicians argue in favor of a gold standard for American currency.] However, for the present, the American dollar bill holds the value that is printed on it, and little more. The only other guarantee on the bill is a Federal Reserve pledge as confirmation in the form of government securities.

23. What does the speaker say about the exact composition of the American dollar bill?

解析：C。录音前半部分提到，纸和墨水的确切成分是政府机密，录音中的the exact composition对应题干中的问题，指的是paper and ink（纸和墨）的确切成分，C项中的government secret为原词复现，well-protected是录音中closely guarded的同义替换，故正确答案为C。

错项排除：录音开头说美元一面是用黑色墨水印刷，一面是用绿色墨水印刷，中间部分说到刚开始发行的绿色钞票，虽然颜色相同，但不能据此判断其成分从开始到现在没有任何变化，故A项可排除。B项利用color进行干扰，录音开头部分只说到，美元钞票的一面用黑色墨水印刷，另一面用绿色墨水印刷，这不能说明美钞的详细成分和颜色有很大关系，故B项错误。D项属于主观臆断，在录音中并无此相关内容，故排除。