Scientists have shown that teens are more active later at night than children and adults, so most teens are probably staying up late for whatever reasons. "The researchers measured the presence of the sleep－promoting hormone (荷尔蒙) in teenagers' saliva (唾液) at different time of the day." They learned that the hormone levels rise later at night than they do in children and adults-and remain at a higher level later in the morning. Measuring the hormone in the saliva is a good way to show that most teens have sleep problems, but changing school schedules isn't the best way to fix this. Most people arguing against changing school schedules would say that it would only cause teens to stay up and wake up later, thus leaving them with the same problem.

　　The biggest problem schools would come across from changing their schedules would be the after－school sports and job conflicts students and teachers would meet. "Teachers reported that students were more alert, and research showed that after－school sports and jobs would suffer." Students in that area might have been able to deal with less time for jobs and sports somehow, but some of them have practice and earlier plans that would become conflicted if this change were to occur. Most students rarely have time to play a sport and do homework before 9 or 10 o'clock. Opening schools later may have worked for some areas, but the reality is often very different.

　　21．The meaning of the underlined word "chaotic" in the first paragraph is close to "________"．

　　A．positive B．apparent

　　C．negative D．absurd

　　答案与解析：C　词义猜测题。根据文章第二段最后两句以及第三段首句的内容可知，改变学校的上课时间会产生负面影响，故此处chaotic意思同negative。

　　22．Most teens have sleep problems because ________.

　　A．they are more active than children and adults

　　B．school work takes up most of their time

　　C．their hormone levels are higher later at night

　　D．they probably like staying up for no reason

　　答案与解析：C　细节理解题。由文章第二段中的内容可知：大多数青少年有睡眠方面的问题是由于他们体内促进睡眠的荷尔蒙含量到深夜才升高。

　　23．Which of the following is TRUE according to the passage?

　　A．There are many conflicts between students and teachers.

　　B．Some of the teachers have practice and earlier plans.

　　C．Most students are able to do sports and homework before 9：00.

　　D. Ways to solve students' sleep problem vary from area to area.

　　答案与解析：D　细节理解题。根据第三段最后一句可得知，解决青少年睡眠不足的问题采取的措施因地区而不同。

　　24．The passage mainly develops by ________.

　　A．giving reasons B．showing examples

　　C．comparing facts D．offering solutions

答案与解析：A　推理判断题。由文章内容可知，这篇文章首先提出观点，然后阐述各个方面的原因，写作方法应是A项。