20．
In many high schools，there are strict rules about using mobile phones．If your phone rings in class，for example，the teacher will be very angry．
Every mobile phone has a ringtone （铃声）．It tells you that someone is calling you．Now，there is a ringtone called"mosquitotone （蚊子铃声）"．Using it，students can receive calls and text messages （短信） in class but the teacher will not realize what is happening．How？
Mosquitotone has a special sound．Only children and teenagers can hear it．As we get older，we are unable to hear some sounds with high frequencies （频率）．As a result，a child or teenager can hear the mosquitotone ringtone，but it's_inaudible_to someone 20or older．
But sometimes，people over 20can hear the ringtone．One British student's phone uses mosquitotone．In class，the student's phone rang．The 32yearold teacher looked at the student 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，says，"Using mosquitotone too often is bad for our hearing．I may use it for pleasure，but not in class．It's unfair to the teachers！"
What do parents think of mosquitotone？"I think it is perfect for places like movie theaters，"says Connie Neeson，44．"Think about it．Yon're watching a film and someone's mobile phone rings．Mosquitotone is good because many people don't hear it．However，it also has a bad influence on their study if students use it in class．"
48．Mosquitotone works becauseD．
A．it is very loud B．it has no sound
C．it has a beautiful sound D．it uses high frequencies
49．The under lined part"it's_inaudible_to"means"B"．
A．it can't be seen by B．it can't be heard by
C．it isn't popular with D．it isn't healthy for
50．The 32yearold teacher felt unhappy because the studentD．
A．was late for class B．didn't do his homework
C．couldn't answer her question D．was using a mobile phone in class
51．Connie Neeson thinks that mosquitotoneC．
A．has no bad influence B．is unfair to the teachers
C．has its advantage and disadvantage D．can't be used in any public place．
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