C

    Waste can be seen everywhere in the school. Some students ask for more food than they can eat and others often forget to turn off the lights when they leave the classroom. They say they can afford these things. But I don't agree with them.

    Waste can bring a lot of problems. Although China is rich in some resources, we are short of others, for example, fresh water. It is reported that we will have no coal or oil to use in 100 years. So if we go on wasting our resources, what can we use in the future and where can we move? Think about it. I think we should say no to the students who waste things every day. Everybody should stop wasting as soon as possible.

    In our everyday life, we can do many things to prevent waste from happening, for example, turn off the water taps when we finish washing, turn off the lights when we leave the classroom, try not to order more food than we need, and so on. Little by little, everything will be changed. Waste can be stopped one day, if we do our best.

阅读下列三篇短文，按要求完成后面提出的问题。

A．蝙蝠在黑暗中能自由地飞翔，用蜡封住其耳朵，虽然把它放在明亮的房间里，仍像喝醉酒一样，一次次地碰到障碍物。后来，物理学家证实了蝙蝠能发出  ①  波，靠这种波的回声来确定目标和距离。

B．如果把八只同样的玻璃杯盛不同深度的水，再用一根细棒依次敲打杯子，可以发现声音的   ②   和水量有关。如果调节适当，可演奏出简单的曲谱，由此我们不难知道古代“编钟”的原理。

C．许多年前，“马可波罗”号帆船在“火地岛”失踪。科学家经过多年的研究，揭开了“马可波罗”号帆船上人们的“死亡之谜”：他们都是死于“亚声”。这是一种人耳听不到的声波，频率低于20 Hz 。当声波的频率在2 ~ 10 Hz时，人会产生恐惧、恶心、头晕等症状，其中5 Hz的声波对人危害最大，这是因为此时人的内脏的固有频率和“亚声”的频率极为相似，会形成内脏的共振，严重时，把内脏振坏而使人丧生。