A、Very impolite. B、Not customary. C、Very urgent. D、Not necessary. C题目问的是根据美国习俗，非常早的电话对于美国人来说意味着什么。由定位句可知，这意味着这件事非常重要，需要立即注意。故选C。

Time talks. It speaks more clearly than words. Time communicates in many ways.
    Consider the different parts of the day, for example. The time of the day when something is done can give a special meaning to the event. Factory managers in the United States fully realize the importance of an announcement made during the middle of the morning or afternoon that takes everyone away from his work. Whenever they want to make an important announcement, they ask." When shall we let them know?"
    In the United States, you don’ t usually telephone someone very early in the morning. If you telephone him early in the day, while he is shaving or having breakfast, the time of the call shows that the matter is very important and requires immediate attention. The same meaning is given to telephone calls made after 11:00 p. m. If someone receives a call during sleeping hours, he assumes it is a matter of life and death. The time chosen for the call communicates its importance.
    Q19: What does the talk mainly talk about?
    Q20: According to the US custom, what does a very early telephone call mean to an American?
    Q21: Which of the following is the best time to phone an American?