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)”.

    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 someone of 20 or older can't.

    But sometimes, people over 20 can hear the ringtone. One British student's phone uses mosquitotone. In class, the student's phone rang. The 22-year-old 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(不公平的) 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. You'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（影响） on their study if students use it in class. ”

 分别将下列各组物质同时加入到足量水中，最终能得到无色、透明溶液的是（　　）

​

菲律宾玛雅生态农场位于首都马尼拉附近，农场主在36公顷的丘陵地经营了稻田和经济林，饲养了2.5万头猪、70头牛和1万只鸭、每天产气近20万立方米的沼气池和饲料加工厂，基本实现了无污染。下图为玛雅农场物质循环生产图，完成下列各题。