A、They must endure wear and tear. B、They must be of the same size. C、They must be made from a rare material. D、They must follow

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, while most European countries prefer a shade of red. That’s almost brown.
    There are no official rules regarding passport colors, 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 degrees Celsius to 50 degrees Celsius, under the conditions where the air can be extremely dry or moist.
    Despite these existing recommendations, there is nothing regarding the cover color. Why? 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. The passports of many countries can take images of their well-known geographical features and wildlife, which also function as security features as some are only visible under a special light.
    Questions 22 to 25 are based on the passage you have just heard.
    22. What do we learn about most European passports?
    23. What does the passage say about the design of passports?
    24. Why are dark colors chosen for passport covers?
    25. What is one special function of the images inside many passports?