题目

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 resu从扬州开往西安的K51次列车，当其自东向西行驶时，车轮对哪侧的铁轨磨损较大 A．南侧  B．北侧  C．东侧  D．西侧