阅读理解

    In many high schools，there are strict rules(规定)about using mobile phones．If your phone rings(响)in class，for example，the teacher will be angry because he thinks you disturb(打扰)him and other students．

    Every mobile phone has a ringtone(铃声)．It tells you that someone is calling you．Now,there is a ringtone called"mosquitotone"．Using it．Students can get calls and messages in class but the teachers will not know what is happening．How?

Mosquitotone has a special sound．Only children and teenagers(青少年)can hear it．As people get older,they are unable to hear some sounds with high frequencies(频率)．As a result，a child or teenager can hear mosquitotone，but someone of 20 or older can't．

    But sometimes,people over 20 can hear mosquitotone．One Brifish boy uses mosquitotone．In class，his phone rang．His maths teacher Aaron Brown looked at him and became unhappy,"I heard that．You know the rule：no mobile phone in class．"

    Not all the students are interested in mosquitotone．Jerry Jones，a high school student of 17，said，"Using mosquitotone too often is bad for our hearing．I may use it for fun．but not in class．It's unfair(不公平的)to the teachers!"

    What do parents think of mosquitotone?"I think it is perfect for places like movie theatres．"says Connie Neeson,44．"Think about it．You're watching a movie and someone's mobile phone rings．Mosquitotone is good because many people don't hear it．However．it also has a bad influence(影响)on their studies if students use it in class．"

根据材料内容选择最佳答案。

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    平生到过不少有名的风景区，但在我的脑子里的印象最深的还是我家乡门前的小溪。春天，春水涨满，桥的两孔像是一对微笑的眼睛。细雨如烟，桥上不时有人打着雨伞走过。对岸的红棉树开花了，燕子在雨中飞来飞去，一阵一阵的风，吹来了断续的笛声……

    面对着巨流滚滚的扬子江，我想起了它的发展的历程。

    最先它不过是雪山冰岩下面滴沥的小泉，逐渐才变成苍苔滑石间的细流，然后是深谷里跳跃着喜悦的白色浪花的溪涧。以后它又逐渐发展，一时它是澄澈的清溪萦回在牛群牧草之间，一时它又是沸腾咆哮、素气云浮的瀑布，一时它是波平如镜、静静地映着蓝天白云的湖泊，一时它又是飞流急湍、奔腾在崇山峡谷之间的险滩。不知经历了多少曲折和起伏，最后它才容纳了许多清的和浊的支流而形成了茫若无涯的、浩浩荡荡的大江。

    我没有到过龙门壶口，没有看到过雁荡龙湫，却看过黄果树的瀑布和许多偏僻地方的大瀑布。

    远离瀑布还好几里。就先听到雷鸣，先看到雾气从林中升起。走前去一看，只见一股洪流直冲而下，在日光映射下，像是悬空的彩练，珠花进发，有如巨龙吐沫；水冲到潭里，激起了沸腾的浪花、晶莹的水泡。大大小小的水珠，随风飘荡，上下浮游，如烟如雾，如雨如尘，湿人衣袖。上有危崖如欲倾坠，下有深潭不可逼视。轰隆的巨响，震耳欲聋，同游旅伴虽想交谈几句，也好像失去了声音。

    看了瀑布使人感到有一股雄壮宏伟的气势，奔腾冲激的力量，云蒸霞蔚的氛围，它虽然没有具体说出什么，但它的冲劲的确使人振奋。    

    小溪流唱着愉快的歌流走了，它将冲击着涯岸流向大海：静静的群山，则仍留在原来的地方，目送那盈盈的水波远去。

    流水一去是决不回来了，但有时也会化作一两片羽云瞭望故乡。

已知a为常数，函数f（x）=x（lnx﹣ax）有两个极值点x₁，x₂（x₁＜x₂）（  ）