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   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 20 or older.
   But sometimes，people over 20 can 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.”

通过“观察种子的形态和结构“的实验，发现玉米和蚕豆种子大小、形状、颜色各不一样．但基本结构相同，它们都具有（　　）

汽车是我们熟悉的交通工具，汽车中应用了许多物理知识，请你运用所学的物理知识解答下列问题：

阅读下面古诗词。
                                                                                   金陵驿①
                                                                                    文天祥
                                                             草合离宫②转夕晖，孤云飘泊复何依！
                                                            山河风景元③无异，城郭人民半已非。
                                                            满地芦花和我老，旧家燕子傍谁飞？
                                                            从今别却江南路，化作啼鹃带血归。
         【注释】①本诗写于1279年，诗人战败被俘，押往大都（北京），途经金陵（南京）。此时南宋已亡，金陵已被攻破四年。②离宫：即行宫，皇帝出巡时临时居住的地方。金陵是宋朝的陪都，所以 有离宫。③元：同"原"。