单选题 Twenty years ago people had one telephone in the house or in the office. Today, people have telephones at home, in cars, in offices, almost everywhere. You can even see people talking into cell phones (手机) on the streets. Once I asked a newspaper reporter for his telephone number. He answered, "Which number do you want? Home number, work number, weekend number, or car number?" Now there are even answering machines, but it seems that nobody likes them. For callers there is the problem of how to talk to them because it is very difficult to have a conversation with a machine. For owners of answering machines, there are problems, too. What kind of message (信息) should be recorded (录音) for the caller? If the message is too short, like "This is an answering matching. Speak now", the caller may not have enough time to get ready. Others may be too long. One long message goes like this: "This is Nathan"s answering machine. Please leave your name, telephone number, what you are like, name of your school or work place and your mother"s first name. Speak now." Probably, people will never call Nathan again.