单项选择题
When some of the stamps are difficult to find, （）
A．nobody will try to get them B．they can become very valuable C．everybody would like to buy them D．they will be worthless
单项选择题 Which of the following statements is true（）
单项选择题 The post office says the stamp is useless when （）
单项选择题 When you post a letter, you must buy（）
