单选题     A stamp is just a piece with a picture and some words printed on one side and some glue on the other side. What makes one of these bits of paper worth any money at all? What makes a ten-fen stamp worth ten fen?
    When you buy a stamp, you also buy service from the post office. You get the letter sent by post. After the stamp has done its work, the post office says it is worthless. You must buy a new one for each letter you send.
    But people often pay money for stamps that have already been used. Stamp collectors have fun just trying to collect as many different kinds as possible. Certain kinds are hard to find. To get one of these uncommon stamps, some collectors are willing to pay a great deal of money. They think it is worth something, and that gives it value. If you collect stamps because they are especially beautiful or tell an interesting story or show all kinds of animals, then those are the ones that have value to you.

单选题     When you post a letter, you must buy ______ .

单选题     The post office says the stamp is useless when ______ .

单选题     When some of the stamps are difficult to find, ______ .

单选题     Which of the following statements is not true? ______