读《渡江战役》图，完成下列问题。

下列材料是1919年6月间上海等地发给北洋政府的部分电函名称，这些电函作为史料能够直接证明

如图所示，固定斜面足够长，斜面与水平面的夹角α=30°，一质量为3m的“L”型工件沿斜面以速度v₀匀速向下运动，工件上表面光滑，下端为挡板．某时，一质量为m的小木块从工件上的A点，沿斜面向下以速度v₀滑上工件，当木块运动到工件下端时（与挡板碰前的瞬间），工件速度刚好减为零，后木块与挡板第1次相碰，以后每隔一段时间，木块就与工件挡板碰撞一次，已知木块与挡板都是弹性碰撞且碰撞时间极短，木块始终在工件上运动，重力加速度为g，求：

根据短文理解，选择正确答案。

    Nowadays, camping has been popular among young people. It's a great way to be outside and enjoy the nature. Whether you prefer to be in the woods, mountains, or by the river, you are sure to have a wonderful experience when you go camping. Here are some suggestions for camping.

    Fire may be dangerous, so just use it only when you must. Make sure to put the fire out when you don't use it. Or it may cause a fire. When you are talking, keep your voice down. If you talk loudly, you may disturb (打扰) other people, and you will also miss the chance to see wild animals outside. It is necessary to put up your tent only in safe areas. If you put up your tent in other areas, you may get into trouble. Remember to do it for your safety. Don't forget to take your camera, or you will miss the wonderful natural wonders. Later, you'll understand how nice they are. Make sure your camera has enough power. People often get lost in the forest. It's a good way to take a map that helps you to get the right direction. Now, people usually take GPS with them. Maps and GPS can help you find the way easily.