A. It had to be purchased by the sender in advance. B. It increased the cost of mail delivery. C. It was difficult to affix to letters. D. It was easy to counterfeit. 参考答案:A 答案解析:句子理解题。以下哪一项被视为邮票的不利之处 A项它必须被发信的人提前购买。B项它增加了信件传递的成本。C项它很难附在信件上。D项它很容易被伪造。