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。

平角的 $\frac{1}{3}$ 是____度，是____角。