听力材料：

　　Jet lag, caused by traveling between time zones, isbecoming a common problem for frequent travelers:for 49 percent it is only a nuisance and for 45percent it is a real problem. [26]It is caused bydisruption to the internal biological clock, and maylead to digestive problems, tiredness, and sleepdisruption. Generally speaking, our biological clock is slightly disturbed if we just move into thenext time zone, but jet lag becomes a problem once we have passed through three or fourtime zones. [27]The body takes about one day to get over each hour of time difference. Butthe seriousness of jet lag problems also depends on our direction of travel. If we travel westwe will be in advance of ourselves as far as our internal clock is concerned, and problems mayarise. A west-to-east journey, on the other hand, makes us late compared to the local time. Itoften demands even greater effort in adjustment since we are not quick enough to catch upwith the new time schedule. Therefore, a trip from New York to Los Angeles often causes fewerproblems than a Los Angeles-New York trip.

　　Questions 26 to 28 are based on the passage you have just heard.

　　26. What's the problem a person suffers from when traveling between time zones?

　　正确答案：A

　　解析：录音开头就提到Jet lag，第2句就说明它的起因是“人体内部的生物钟被打乱”，故答案为A。

　　27. How many hours does one need to get over his jet lag problem if he travels across threetime zones?

　　正确答案：D

　　解析：数字题，一般要做记录并进行简单的运算。原文说到1个小时的时差需要1天来调整，那么3个时区就是3个小时的时差，所以应该用3天即72个小时，D正确。

　　28. What's the main purpose of the passage?

　　正确答案：A

　　解析：主旨题。录音的话题是jet lag problems，但只是客观陈述现象和原因，并未提及避免的方法，故A对而B错。

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