Question 24 is based on the passage you have just heard.

听力原文    

        Maybe you carry the most powerful passport in the world, or perhaps the rarest passport. Whatever the case, your passport will likely be one of four colors: red, blue, green or black, although there are slight variations. Switzerland’s passport is dark red, for example, (22) [while most European countries prefer a shade of red that’s almost brown.] There are no official rules regarding passport colors, (23) [but countries follow a certain set of norms when designing them.] The International Civil Aviation Organization, which works to define the principles of air travel, suggests that countries use a suitable type, size and style for official documents like passports. There are also a few compulsory aspects of passports. They must be made from a material that bends. They should be able to be read by a machine at temperatures ranging from 10℃ to 50℃ and conditions where the air can be extremely dry or moist. Despite these existing recommendations, there is nothing regarding the cover color. Why? (24) [Simply put, countries stick to darker and more conservative colors because they appear more official] and they can also hide dirt and wear. While there’s little innovation when it comes to passport cover color, the same cannot be said for the pages. (25) [The passports of many countries contain images of their well-known geographical features and wildlife, which also function as security features as some are only visible under a special light.]

24. Why are dark colors chosen for passport covers? （为什么护照封面要选深色？）

解析：B。录音中提到，对于护照封面，各国都坚持使用较深、较保守的颜色，因为它们看起来更正式，B项中的more official为原词复现，故B项正确。

错项排除：录音中提到各国护照都坚持使用较深、较保守（more conservative）的颜色，但这里的conservative形容的是颜色，而不是封面选用深色的目的，故排除A项。C、D两项在录音中未提及，故均可排除。